dts: nxp mcxc: Configure boot source to flash

NXP mcxc series has boot source configured to ROM. ROM bootloader
waits 5 sec for active peripheral detection timeout before jumping
to application in flash which makes booting very slow.
Change configuration to boot from flash and allow boot source
selection by external pin.

Signed-off-by: Michal Smola <michal.smola@nxp.com>