mfd: pm8xxx-pwm: Add support for pwm size of 7 bit

The existing pwm driver only supports 6 and 9bit pwm modes.
In 6 bit and 9 bit PWM modes, PWM high bank supports 12mA current
sink and 4mA current sink is not supported with PWM low bank in
these PWM podes. To support 4mA PWM current sink enable 7 bit PWM
mode and PWM low bank.

CRs-Fixed: 486318
Change-Id: I639d516196bda47346c979f5b8ae3c27fcaef104
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>