mfd: Move TPS6507x register definition to header file.

Other sub-drivers for the TPS6507x chip will need to use register
definition so move it out of the source file and into a header file.

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>