msm: krait-regulator: add PWM mode settling time

A settling time of 50 us is needed when switching from PFM mode
to PWM mode. Phase settling time needs to be 50 us too.

CRs-Fixed: 479580
Change-Id: Ieb6ff7536dabe4ad02cc6724719a0aadd77a5068
Signed-off-by: Willie Ruan <wruan@codeaurora.org>