soc: nxp: Add Power Management support for RT5xx

1. Support Sleep, Deep Sleep and Deep Power down modes
2. Enable the MEMC FlexSPI driver when using device power
   management so we can reconfigure the FlexSPI pins to
   save power. The MEMC FlexSPI driver is enabled when we
   enable the Flash subsystem, however we would like to
   reconfigure the FlexSPI pins even when the Flash driver
   is disabled, hence MEMC is selected when PM_DEVICE
   is turned on.

Signed-off-by: Mahesh Mahadevan <mahesh.mahadevan@nxp.com>