irqchip/csky-mpintc: Fixup mask/unmask implementation

The mask/unmask must be implemented, and enable/disable supplement
them if the HW requires something different at startup time. When
irq source is disabled by mask, mpintc could complete irq normally.

So drop enable/disable if favour of mask/unmask.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211101134534.3804542-1-guoren@kernel.org