mfd: devicetree: bindings: Document PM8921/8058 PMICs

PM8921 and PM8058 are PMICs found paired with MSM8960 and MSM8660
devices respectively. They contain subdevices such as keypads,
RTC, regulators, clocks, etc.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>