samples: subsys: nvs: add testcase for MCXN947 internal flash

For boards frdm_mcxn947 and mcx_n9xx_evk test NVS in internal flash in
addition to external QSPI flash.  Also enable the NVS subsystem on
these boards for Twister.

Signed-off-by: Derek Snell <derek.snell@nxp.com>