powerpc: Rework PPC_RAW_xxx() macros for prefixed instructions

At the time being, we have PPC_RAW_PLXVP() and PPC_RAW_PSTXVP() which
provide a 64 bits value, and then it gets split by open coding to
format it into a 'struct ppc_inst' instruction.

Instead, define a PPC_RAW_xxx_P() and a PPC_RAW_xxx_S() to be used
as is.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5d146b31b943e7ad674894421db4feef54804b9b.1621506159.git.christophe.leroy@csgroup.eu