ARM: dts: msm: Describe DT bindings for gang leader

On certain MSMs the Kraits could be powered by a single supply line from
the PMIC. Inside the PMIC this single supply line is powered by multiple
regulators operating in ganged mode. There is a gang leader which
controls the voltage and phases of the single supply line.

The Kraits access the gang leader via the L2 SPM. However, the SPM limits
the accesses to only four PMIC registers.  This device and the associated
driver supplement the Krait/L2 SPM by allowing them access to other
registers in the gang leader. The driver is primarily used to implement
workarounds for various PMIC chips where the ganged mode is enabled.

Change-Id: I50f0e96835f4ece173be790efb170fe145ac2435
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>