ARM: OMAP4: PM: Add WakeupGen module as OMAP gic_arch_extn

OMAP WakeupGen is the interrupt controller extension used along
with ARM GIC to wake the CPU out from low power states on
external interrupts.

The WakeupGen unit is responsible for generating the wakeup event
from the incoming interrupts and enable bits. It is implemented
in the MPU always ON power domain. During normal operation,
WakeupGen delivers the external interrupts directly to the GIC.

WakeupGen specification has one restriction as per Veyron version 1.6.
It is SW responsibility to program interrupt enabling/disabling
coherently in the GIC and in the WakeupGen enable registers. That is, a
given interrupt for a given CPU is either enable at both GIC and WakeupGen,
or disable at both, but no mix. That's the reason the WakeupGen is
implemented as an extension of GIC.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Jean Pihet <j-pihet@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Tested-by: Vishwanath BS <vishwanath.bs@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>