mfd: Allow configuration of VDCDC2 for tps65010

Add function to allow the configuation fo the VDCDC2 register by
external users, to allow changing of the standard and low-power
running modes.

This is needed, for example, for the Simtec IM2440D20 where we need
to use the low-power mode to shutdown the LDO/DCDC that are not needed
during suspend (saving substantial power) and the runtime use of the
low-power mode to change VCore.

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Simtec Linux Team <linux@simtec.co.uk>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>