powerpc/pseries: Drop support for MPIC in pseries

MPIC was only used by Power3 which is now unsupported, so drop support
for MPIC. XICS is now the only supported interrupt controller for
pSeries so make the XICS functions generic.

Signed-off-by: Rashmica Gupta <rashmicy@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>