ASoC: rt9120: Use pm_runtime and regcache to optimize 'pwdnn' logic

From the datasheet, the only way to meet the lowest power consumption is
to pull low the 'pwdnn' gpio. But if it is low, IC will keep in
reset state, all registers reset to default.

And the power consumption is listed below
1. amp off and 'pwdnn' high => idle state, PVDD = 1mA, DVDD = 7mA
2. amp off and 'pwdnn' low  => shutdown state, PVDD < 20uA, DVDD < 15uA

It's the large difference for the consumption current

This fix is to use pm_runtime and regcache to handle 'pwdnn' gpio
control.

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Link: https://lore.kernel.org/r/1652926418-8519-3-git-send-email-u0084500@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>