slim_ngd: Ensure device state consistency with runtime-pm during SSR

During ADSP SSR, client's thread calling runtime-pm resume will
fail since ADSP is unreachable. Make sure such power-up/resume attempt
is blocked until ADSP comes up by using mutex during device-state
transition.

Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>