boards: rd_rw612_bga: flash size with monolithic feature

When building with monolithic feature enabled, the firmwares binaries
will be linked during the build with the application. This means we can
remove the application size limit due to the firmware storage in flash
not useful anymore.

Signed-off-by: Axel Le Bourhis <axel.lebourhis@nxp.com>
Signed-off-by: Yassine El Aissaoui <yassine.elaissaoui@nxp.com>