sel4bench.h: update documentation and style

* Expand documentation of `sel4bench_enable_counters()`.
* Document `sel4bench_get_num_counter_chunks()`.
* Add missing @param and @return notations.
* Consistently use plurals in the strings for generic event names.
* Use Markdown backticks for literals (e.g., symbol names).
* Keep @param and @return descriptions terse (they should be noun
phrases appropriate for tool tips). Elaborations go in the function
description.
* Don't abbreviate `sel4bench_init()` and `sel4bench_destroy()` as
`_init()` and `_destroy()`, respectively. Use the full names.
* Move documentation of constraint on number of counters to
`sel4bench_get_num_counters()`, instead of duplicating it in two other
functions.
* Emphasise that the cycle counter is not a performance counter.
* Clarify reference to ARM1136 platform, a.k.a. "kzm".

Flag open questions:
* Should some of these prototypes and constant objects be moved to a
"private" header file?

Minor style issues:
* Perform minor wordsmithing.
* Use hyphens more idiomatically.
* Wrap lines at 80 columns.
* Use two spaces between sentences for readability.

trivial: style changed

sel4bench: make it clear reset_counters is not all

sel4bench_reset_counters does not reset the cycle counter

Fix licenses

sel4bench: reset all counters

Resetting a subset of the counters a) doesn't work on all platforms c)
doesn't make much sense and b) is less code, d) unused functionality

If this functionality is required it can be readded in a working form at
a later date.

sel4bench: add missing utils/arith.h

sel4bench: helpers for looping through events

This commit adds helpers in sel4bench.h to loop through a set of
events when the number of events you want to read are greater than the
number of counters provided by the platform.

These can be used for the generic events or for a custom set
of events.

sel4bench: s/COUNTERS/EVENTS in number of events

sel4bench: generic events name and constant sanity

Add a static assert to make sure they are at least the same length

sel4bench: Add MEMORY_ACCESS as generic event

I mistakenly deleted it as platforms variably defined
this counter spelling MEMORY or MEM in the constant.

sel4bench: names for generic events

sel4bench: style

sel4bench: use ccnt_t and remove duplicate

Was duplicated sel4bench_counter_t and format function

sel4bench: remove SEL4BENCH_API indirection macro

Use UNUSED and static directly

sel4bench: improve interface

- define array of generic events
- drop generic events that don't exist for all platforms (i.e are not
generic)
- define number of generic events
- define types to illustate the difference between a counter and an
event

sel4bench: use #pragma once

sel4bench: fix include structure and licenses

For too long sel4bench has relied on relative includes
and filenames with the CPU/ARMV in it.

Finally fix it.

sel4bench: Removed SEL4BENCH_FENCE macro

This macro has been moved to libutils.

libsel4bench: Add performance counter descriptions for runtime display

sel4bench: move FENCE macro from app to lib.

This could be useful elsewhere.