mfd: tps6586x: Implement irq_set_wake

rtc-tps6586x calls enable/disable_irq_wake() during suspend/resume. Since
the main tps6586x irq_chip doesn't implement .irq_set_wake, this causes
the RTC's enable_irq_wake() to fail, and the disable_irq_wake() to spew a
WARN about unbalanced wake disable. Solve this by implementing
.irq_set_wake.

Also, I assume that enable_irq_wake() shouldn't be called unconditionally
in tps6586x_irq_init(), since this is now triggered by IRQ children
setting up their cascaded IRQs for wake. So, remove that.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>