[POWERPC] Remove fixup_bigphys_addr() for arch/powerpc to avoid link error

There are no actual implementations of fixup_bigphys_addr() in
arch/powerpc, and with a 64-bit aware ioremap() and so forth, it
should no longer be necessary.  This patch removes the last dregs of
fixup_bigphys_addr() from arch/powerpc.

In fact, the only reason this hasn't caused link errors already is
that nobody must have tried using one of the small number of drivers
using io_remap_pfn_range() on one of the small number of platforms
which are 32-bit but define CONFIG_PHYS_64BIT.  Nonetheless this fixes
a bug, and should go into 2.6.22.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>