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>
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I'm tempted to Cc stable here, since this bug is certainly present in
older kernel releases. However, the only user-visible aspect is the WARN
on resume, so I'm not sure if it's worth it?