selftests/powerpc: Add a rtas_filter selftest

Add a selftest to test the basic functionality of CONFIG_RTAS_FILTER.

Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
[mpe: Change rmo_start/end to 32-bit to avoid build errors on ppc64]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200820044512.7543-2-ajd@linux.ibm.com