s390: remove virt_to_phys implementation

virt_to_phys on s390 currently uses the LRA instruction to translate
virtual to physical addresses. This creates an unnecessary overhead
and caused trouble with dma debugging code (when called with an
address pointing to a already unmapped page).
Just get rid of s390's implementation and use the one from
asm-generic/io.h .

Note: with this change virt_to_phys will no longer work on vmalloc'ed
addresses.

Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>