dt-bindings: regulator: Add optional RPM-SMD regulator properties

Add "qcom,min-dropout-voltage" to specify the min dropout voltage
for RPM-SMD regulators, "qcom,supported-modes" to specify supported
BOB regulator modes and "qcom,mode-threshold-currents" to specify
minimum allowed currents for supported BOB modes.

Change-Id: Ife7ce8d1e506b41fce8079b0424fbf71dc874d50