mfd: pmic8901: 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: Ia95894a6a032a254b7ca3f8a251b4984e1432818
Signed-off-by: David Collins <collinsd@codeaurora.org>