mfd: devicetree: bindings: Add Qualcomm RPM regulator subnodes

Add the regulator subnodes to the Qualcomm RPM MFD device tree bindings.

Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>