mfd: Add db8500-prcmu accessors for PLL and SGA clock

This extends the DB8500 PRCMU driver with accessor calls for the
PRCMU PLL and SGA clocks.

Signed-off-by: Mattias Nilsson <mattias.i.nilsson@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>