powerpc/embedded6xx/hlwd-pic: Prevent interrupts from being handled by Starlet

The interrupt controller inside the Wii's Hollywood chip is connected to
two masters, the "Broadway" PowerPC and the "Starlet" ARM926, each with
their own interrupt status and mask registers.

When booting the Wii with mini[1], interrupts from the SD card
controller (IRQ 7) are handled by the ARM, because mini provides SD
access over IPC. Linux however can't currently use or disable this IPC
service, so both sides try to handle IRQ 7 without coordination.

Let's instead make sure that all interrupts that are unmasked on the PPC
side are masked on the ARM side; this will also make sure that Linux can
properly talk to the SD card controller (and potentially other devices).

If access to a device through IPC is desired in the future, interrupts
from that device should not be handled by Linux directly.

[1]: https://github.com/lewurm/mini



Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>