mfd: Add tps6507x board data structure

Add mfd structure which refrences sub-driver initialization data. For example,
for a giving hardware implementation, the voltage regulator sub-driver
initialization data provides the mapping betten a voltage regulator and what
the output voltage is being used for.

Signed-off-by: Todd Fischer <todd.fischer@ridgerun.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>