irqchip/irq-bcm7120-l2: Use correct I/O accessors for irq_fwd_mask

Initialization of irq_fwd_mask was done using __raw_writel() which
happens to work for all cases except when using ARM BE8 which requires
writel() (with the proper swapping). Move the initialization of the
irq_fwd_mask till later when we have correctly defined our I/O
accessors.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>