mfd: Add comments for the twl4030-power register and bit layout

Describe how the resource registers are laid out and the various bit-fields in
them.

Signed-off-by: Amit Kucheria <amit.kucheria@verdurent.com>
Cc: linux-omap@vger.kernel.org
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>