irqchip/mtk-sysirq: Extend intpol base to arbitrary number

Originally driver only supports one base. However, MT6797 has
more than one bases to configure interrupt polarity. To support
possible design change, here comes a solution to use arbitrary
number of bases.

Signed-off-by: Mars Cheng <mars.cheng@mediatek.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>