scsi: ufs: fix ufs power down specs violation in shutdown path

As per specs, while powering off the ufs device, RST_N signal
and REF_CLK signal should be deasserted between VSS(Ground)
and VCCQ.

As per current implementation during power down, we follow
below sequence of events:

1. Turn-off VCCQ
2. Turn-off VCC
3. Turn-off REF_CLK

It is observed that during powering down of ufs device,
REF_CLK is taking longer than usual to ramp down which
keeps REF_CLK signal to be above VCCQ and hence causing
specs violation during power down.

Similarly since RST_N is not asserted as part of power down,
RST_N signal is not falling between VSS and VCCQ and hence
causing specs violation for RST_N as well.

To guarantee that, we should follow below sequence of
events during power down in shutdown path:

1. Turn-off REF_CLK
2. Assert RST_N low
3. Turn-off VCCQ
4. Turn-off VCC.

Change-Id: I69dcec71a136cc3d1673c523dc73575cbbbcac4e
Signed-off-by: Nitin Rawat <nitirawa@codeaurora.org>