mfd: Generically describe interactions with the DB8500 PRCMU

There is only one method used to communicate with the DB8500 PRCMU, via I2C.
Now this can be assumed, there is no requirement to specify the protocol in
the function name. This patch removes protocol specifics and uses a more
generic naming convention.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>