arch: arm: cortex_a_r: Fix mrc/mcr instruction usage

The coprocessor number in ARM `mrc` and `mcr` instructions must be prefixed
with `p`.

GNU assembler allows specifying coprocessor number without the `p` prefix;
but, LLVM assembler is more picky about this and prints out "invalid
instruction" error otherwise.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>