x86: asm/io.h: unify virt_to_phys/phys_to_virt

Impact: unify identical code

asm/io_32.h and _64.h has functionally identical definitions for
virt_to_phys, phys_to_virt, page_to_phys, and the isa_* variants, so
just unify them.

The only slightly functional change is using phys_addr_t for the
physical address argument and return val.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>