irqchip/mmp: Do not use of_address_to_resource() to get mux regs

The "regs" property of the "mrvl,mmp2-mux-intc" devices are silly. They
are offsets from intc's base, not addresses on the parent bus. At this
point it probably can't be fixed.

On an OLPC XO-1.75 machine, the muxes are children of the intc, not the
axi bus, and thus of_address_to_resource() won't work. We should treat
the values as mere integers as opposed to bus addresses.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Link: https://lore.kernel.org/r/20190822092643.593488-7-lkundrak@v3.sk