boards: nxp: frdm_mcxn947: use core argument to target CPU0

In future versions of Linkserver, specifying the core argument within
the device string will not be supported. Therefore, move the FRDM
MCXN947 board to specify the core directly instead of using the device
string.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>