powerpc/powernv: Allow manually invoking special reboots

OPAL provides several different kinds of reboot for the kernel to use,
namely forcing a full reboot, platform error reboot and MPIPL. Right now
triggering the alternative resets requires some ad-hoc method such as
triggering a kernel crash and hoping the stars align. It's sometimes handy
to be able to trigger one of these resets directly, so add a way to do
that.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191101085522.3055-2-oohall@gmail.com