dt-bindings: regulator: Add snapshot of rpm-smd-regulator doc

This is snapshot of the rpm-smd-regulator.txt as of msm-4.14
'commit 637a26bd203d34ce("regulator: rpm-smd: Add support for
set_load function.")'.

Change-Id: Ibcd863e65238d1ff10aee235b570000e698321b6