PCI: apple: Move away from INTMSK{SET,CLR} for INTx and private interrupts

T602x seems to have dropped the rather useful SET/CLR accessors
to the masking register.

Instead, let's use the mask register directly, and wrap it with
a brand new spinlock. No, this isn't moving in the right direction.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Tested-by: Janne Grunau <j@jannau.net>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Link: https://patch.msgid.link/20250401091713.2765724-8-maz@kernel.org