dt-bindings: mfd: Document STPMIC1

STPMIC1 is a PMIC from STMicroelectronics. The STPMIC1 integrates 10
regulators, 3 power switches, a watchdog and an input for a power on key.

Signed-off-by: Pascal Paillet <p.paillet@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>