soc: nxp_rw6xx: Add support for Power Mode 3

This maps to Zephyr power state Standby. In this power
state the OS Timer cannot be used as a wakeup source as
it will be powered off. Hence the counter is enabled
and RTC is used to keep track of system ticks and wakeup
the system.

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