x86: unify kernel_physical_mapping_init() function signatures

Impact: cleanup

In preparation for moving the function declaration to a header file,
unify 32-bit and 64-bit signatures.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <1236257708-27269-16-git-send-email-penberg@cs.helsinki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>