powerpc: Use SPRN_xxx rather than xxx for SPR numbers

This changes symbols like HID0, SPRG3, SRR0, SRR1 etc. that refer to
special purpose registers to SPRN_HID0, SPRN_SPRG3, etc.  Using the
SPRN_ symbols clutters the namespace less, and the forthcoming merge
of asm/processor.h and asm/reg.h is going to remove the non-SPRN_
versions.

Signed-off-by: Paul Mackerras <paulus@samba.org>