mfd: add set_ts_bits for pcap

Some TS controller bits are on the same register as the ADC control, save
TS specific bits and export a set_ts_bits function so the TS driver can set
it with the adc_mutex lock held.

Signed-off-by: Daniel Ribeiro <drwyrm@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>