genirq: Add IRQCHIP_SKIP_SET_WAKE flag

Some irq chips need the irq_set_wake() functionality, but do not
require a irq_set_wake() callback. Instead of forcing an empty
callback to be implemented add a flag which notes this fact. Check for
the flag in set_irq_wake_real() and return success when set.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Thomas Gleixner <tglx@linutronix.de>