irqchip/aspeed-scu-ic: Correctly initialise status and enable registers

The status and enable registers are never initialised with sensible
default values. Fix those.

Signed-off-by: Ryan Chen <ryan_chen@aspeedtech.com>
[maz: commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230130085430.635583-1-ryan_chen@aspeedtech.com