arm64: Constify hwcap name string arrays

The hwcap string arrays used for generating the contents of
/proc/cpuinfo are currently arrays of non-const pointers.

There's no need for these pointers to be mutable, so this patch makes
them const so that they can be moved to .rodata.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>