arm64: Drop generic xlate_dev_mem_{k,}ptr()

The code that provides /dev/mem uses xlate_dev_mem_{k,}ptr() to
avoid making a cachable mapping of a non-cachable area on ia64.
On arm64 we do this via phys_mem_access_prot() instead, but provide
dummy versions of xlate_dev_mem_{k,}ptr().

These are the same as those in asm-generic/io.h, which we include from
asm/io.h

Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>