powerpc: mark 64-bit PD_HUGE constant as unsigned long

When compiled for 64-bit, the PD_HUGE constant is a 64-bit integer.
Mark it as an unsigned long.

This squashes over a thousand sparse warnings on my minimal T4240RDB
(e6500, ppc64be) config, of the following 2 forms:

arch/powerpc/include/asm/hugetlb.h:52:49: warning: constant 0x8000000000000000 is so big it is unsigned long
arch/powerpc/include/asm/nohash/pgtable.h:269:49: warning: constant 0x8000000000000000 is so big it is unsigned long

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>