mfd: db8500-prcmu: Support platform dependant device selection

The main aim for this cycle is to have the u8540 booting to a
console. However, the u8540 doesn't support all of the u8500
platform devices yet. After this stage is complete we can then
fill in the inadequacies, such as specific clock support at a
later date. To achieve this we're placing devices supported by
all platforms into a common device structure and the remaining
ones into a platform specific one.

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