slim_ngd: Use DSP before-shutdown notification for DSP SSR

QMI_SERVER_EXIT notification comes too late (after the DSP has
shutdown). This can result in unclocked access by BAM if there was
in-flight transaction. Use BEFORE_SHUTDOWN notification of SSR
framework instead to make sure in-flight transaction is finished
before DSP turns off its resources.

Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>
Signed-off-by: Dilip Kota <c_dkota@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
Signed-off-by: DerRomtester <dieb.stefan.96@gmail.com>

Conflicts:
	drivers/slimbus/slim-msm-ngd.c