selftests/powerpc: Add matrix multiply assist (MMA) test

Adds a simple test of some basic matrix multiply assist (MMA)
instructions.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Tested-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200622021832.15870-1-alistair@popple.id.au