irqchip/imx-intmux: Implement intmux runtime power management

When the system is suspended, we can explicitly disable clock to save
power. To achieve this, we need save registers' state since it could be
lost after power off.

Implement power management which will:
- Turn the clock off after probing
- Disable clock and save registers' state on system suspend, as
  well as enable clock and restore registers' state on resume
- Rely on the Power Domain framework to shutdown the intmux
  power domain

Without CONFIG_PM, the clock is always on after probe stage.

Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
[maz: revamped commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200727141734.24890-2-qiangqing.zhang@nxp.com