powerpc: Add ICSWX instruction

Add the asm ICSWX and ICSWEPX opcodes.  Add definitions for the
Coprocessor Request structures needed to use the icswx calls to
coprocessors.  Add icswx() function to perform the ICSWX asm
using the provided Coprocessor Command Word value and
Coprocessor Request Block structure.

This is required for communication with the NX-842 coprocessor on
a PowerNV system.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>