ssb: Implement PMU LDO control and use it in b43

Implement the "PMU LDO set voltage" and "PMU LDO PA ref enable"
functions, and use them during LP-PHY baseband init in b43.

Signed-off-by: Gábor Stefanik <netrolller.3d@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>