The PM8xxx family of PMICs contain an RTC.  This RTC is described as a
subnode of the PM8xxx.  Document these bindings, and replace the pwrkey
node in the example with the RTC, which is now described in this
document.

While we're here, add a short description to the device tree bindings
describing what the the PM8xxx devices are and how they are expected to
be used.
Signed-off-by: NJosh Cartwright <joshc@codeaurora.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>