powerpc: Replace PPC_INST_NOP by PPC_RAW_NOP()

On the road to removing all PPC_INST_xx defines in
asm/ppc-opcodes.h, change PPC_INST_NOP to PPC_RAW_NOP().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ad46c195ca1b8572629ef07ba6bfe247585239a6.1621506159.git.christophe.leroy@csgroup.eu