boards: frdm_mcxn947: Add a variant for executing from QSPI

1. The boards always boots from internal flash. However it does
have an external QSPI that can be used to store Zephyr.
Add a variant to support running Zephyr from QSPI Flash.
Running Zephyr from QSPI requires a bootloader like MCUboot
to be pogrammed to internal flash.
2. Create a common dts file to add features that are enabled for
CPU 0 that is included by both vairants i.e internal flash (default)
and QSPI variant.

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