regulator: add pcap driver

Add (partial) support for the voltage regulators on the PCAP2 PMIC.

Signed-off-by: Daniel Ribeiro <drwyrm@gmail.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>