mfd: db8500-prcmu: Add devicetree bindings

This driver was merged in the early days of device tree
on Arm in 2012 and somehow we failed to provide bindings
for it. Fix it up with some YAML bindings.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>