powerpc/pseries: Select ARCH_RANDOM on pseries

We have a driver for the ARCH_RANDOM hook in rng.c, so we should select
ARCH_RANDOM on pseries.

Without this the build breaks if you turn ARCH_RANDOM off.

This hasn't broken the build because pseries_defconfig doesn't specify a
value for PPC_POWERNV, which is default y, and selects ARCH_RANDOM.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>