powerpc: Fix 8xx build failure

The 'powerpc ioremap_prot' broke 8xx builds:

include2/asm/pgtable-ppc32.h:555: error: '_PAGE_WRITETHRU' undeclared (first use in this function)
include2/asm/pgtable-ppc32.h:555: error: (Each undeclared identifier is reported only once
include2/asm/pgtable-ppc32.h:555: error: for each function it appears in.)

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>