mfd: pm8xxx-misc: Add delay after disabling regulators in shutdown function

Add delay after disabling each FTSMPS regulator in
__pm8901_reset_pwr_off.  Add additional delay between disabling the
last regulator and when PS_HOLD is lowered to shutdown or restart
the system.

This delay is necessary to limit transient loading on LDO 7 and
dVdd regulators during the PMIC power off sequence.

Change-Id: I0a833f47164cf7c4ba1d0ae1f73c14b595908b3a
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>