mte_enable_kernel_*() are not needed if KASAN_HW is disabled.

Add ash defines around the functions to conditionally compile the
functions.
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20210315132019.33202-7-vincenzo.frascino@arm.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>