mfd: pm8921-core: Add support for PMIC PM8917 chip

The PMIC PM8917 chip is very similar to the PM8921 chip. Modify
the pm8921-core driver so that it can also handle PM8917 chip
which has different number of gpios and mpps, and no LPG/PWM,
no LEDs and no vibrator.

Change-Id: I252adc2ddc4bcd9bb8b8fb5113444356ba7c969e
Signed-off-by: Willie Ruan <wruan@codeaurora.org>