libplatsupport: fix previous reverts

Due to my messing up of changelists, the revert
did not go through as cleanly as I'd hoped.

This commit reinstates
"libplatsupport: riscv plic irqchip parser"
23092562396ae9209679c83d75d25a953b70c2c6
and picks changes necessary from
"libplatsupport: hifive ltimer refactor"
c0c8ddcfac95a09b50e82cfa88367145d7aeaa5e
in order to build
"libplatsupport: refactor more timers"
ef1dfb7da52b8324281ee6999951bf603d44bdd7

Revert "libplatsupport: hifive ltimer refactor"

This reverts commit c0c8ddcfac95a09b50e82cfa88367145d7aeaa5e.

libplatsupport: hifive ltimer refactor

- Introduce two new helper functions
`helper_fdt_alloc_simple`, `create_ltimer_simple`
These are only used when there's 1 reg and 1 irq
to register.
- Refactor hifive timers to use these helper functions.
- Nit: init time_h explicitly instead of being calloc'd

libplatsupport: Fix typo in the FDT helpers

The definition for the register mapping helper function had used an
'size_t' for the offset parameter, but the prototype uses an 'unsigned
int'. This commit fixes the definition so that it matches the prototype.

libplatsupport: Add helper functions to FDT

These new helper functions in the FDT interface are mostly convenience
wrappers over the existing register and IRQ walking functions. Instead
of providing a callback, the user can present an offset and the helper
functions will attempt to map that particular register block or
interrupt.

libplatsupport: Use const for static strings

This is more compatible with existing interfaces.

libplatsupport: Fix locate interrupt-parent logic

The interrupt-parent property can be found in nodes that generate
interrupts and not only in their parent nodes.

libplatsupport: Look for root interrupt controller

The first interrupt parent that we find may just be a interrupt
forwarder and not the root interrupt controller. This commit adds logic
to look for the root interrupt controller from a interrupt forwarder.

libplatsupport: Fix wrong function call

The wrong function was called to get the length of the 'size-cells'
property of the target device node in the FDT.

trivial: Fix compiler 'const' warnings

libplatsupport: Add FDT parsing library

This FDT parsing library is a collection of utility functions which
parses the FDT with the help of libfdt to simplify the common use cases
of the FDT.