rtc: omap: Support ext_wakeup configuration

Support configuration of ext_wakeup sources. This patch makes it
possible to enable ext_wakeup and set it's polarity, depending on board
configuration. AM335x's dedicated PMIC (tps65217) uses ext_wakeup to
notify about power-button presses. Handling power-button presses enables
to recover from RTC-only power states correctly.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>