soc: mediatek: add a fixed wait for SRAM stable

soc: mediatek: add a fixed wait for SRAM stable

MT7622_POWER_DOMAIN_WB doesn't send an ACK when its managed SRAM becomes
stable, which is not like the behavior the other power domains should
have. Therefore, it's necessary for such a power domain to have a fixed
and well-predefined duration to wait until its managed SRAM can be allowed
to access by all functions running on the top.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Weiyi Lu <weiyi.lu@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>