Eliminate one of the causes of spurious interrupts on armv6. The arm weak

Eliminate one of the causes of spurious interrupts on armv6.  The arm weak
memory ordering model allows writes to different devices to complete out
of order, leading to a situation where the write that clears an interrupt
source at a device can complete after a write that unmasks and EOIs the
interrupt at the interrupt controller, leading to a spurious re-interrupt.

This adds a generic barrier function specific to the needs of interrupt
controllers, and calls that function from the GIC and TI AINTC controllers.
There may still be other soc-specific controllers that need to make the call.

Reviewed by:	cognet, Svatopluk Kraus <onwahe@gmail.com>
MFC after:	3 days