regulator: rpm: add support for RPM-controller SMB208

The IPQ8064 reference boards make use of SMB208 regulators which are
controlled by RPM.  Implement support for these regulators in the RPM
regulator driver.

Signed-off-by: Josh Cartwright <joshc@codeaurora.org>
Acked-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Mark Brown <broonie@kernel.org>