[POWERPC] Remove ioremap64 and fixup_bigphys_addr

In order to suppose platforms with devices above 4Gb on 32 bits platforms
with a >32 bits physical address space, we used to have a special ioremap64
along with a fixup routine fixup_bigphys_addr.

This shouldn't be necessary anymore as struct resource now supports 64 bits
addresses even on 32 bits archs. This patch enables that option when
CONFIG_PHYS_64BIT is set and removes ioremap64 and fixup_bigphys_addr.

This is a preliminary work for the upcoming merge of 32 and 64 bits io.h

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>