irqchip/gic-v3-mbi: Fix uninitialized mbi_lock

The mbi_lock mutex is left uninitialized, so let's use DEFINE_MUTEX
to initialize it statically.

Fixes: 50528752

 ("irqchip/gic-v3: Add support for Message Based Interrupts as an MSI controller")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>