riscv: Add UNUSED tags

These variables are unused in certain build configurations.

riscv: Use strnlen over strlen

strlen cannot currently go through the verification framework, so we shall opt to use
strnlen with a sufficiently large maxlen.

riscv: Use strncmp instead of strcmp

strcmp is difficult to prove correspondence with in verification and it is easier to
replace with strncmp for the moment.

riscv: Remove completed todo

riscv: Move strcmp to util

riscv: Move strlen to util

riscv: Replace strings with char arrays

The C parser does not understand string literals. This is unfortunate, but necessary.

riscv: Fill physical memory regions from FDT

Removes the hardcoded physical memory region from the spike platform and fills it from
FDT parsing instead.

riscv: Cleanup FDT code

Rewrites the FDT code to remove all the disallowed function pointers have attempt to perform
the equivalent functionality of query_mem in place in the main parser. With the callback
system gone all the rest of the unused FDT code is deleted as it doesn't make sense.

riscv: Cleanup fdt code

This deletes a bunch of unused fdt code that does not make sense on seL4 and crusesh
warnings in the rest.
Also documents some further cleanup that needs to happen.

riscv: Guard fdt printing code

This code relies on vfprintf definitions that are only compiled if CONFIG_PRINTING
is enabled

RISC-V: Port FDT from riscv-pk (priv-1.10)