irqchip/irq-mtk-cirq: Move register offsets to const array

In preparation to add support for new SoCs having a different
register layout, add an enumeration that documents register
offsets and move the definitions for the same to a u32 array;

Selecting the right register offsets array is done by adding an
of_device_id array containing all of the currently supported
compatible strings pointing to the "v1" offsets array (as data):
since no devicetree declares the `mediatek,mtk-cirq` compatible
without a SoC-specific one, it wasn't necessary to provide any
legacy fallback.

Every usage of the aforemementioned definitions was changed to
get a register address through a newly introduced `mtk_cirq_reg()`
accessor.

This change brings no functional changes.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221128092217.36552-4-angelogioacchino.delregno@collabora.com