zephyr: Add NRF54L configuration

Adds default Kconfig configuration that allows to build MCUboot
for NRF54L.
Currently this configuration turns off WDT and FPROTECT,
which is TODO to fix.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
Signed-off-by: Mateusz Michalek <mateusz.michalek@nordicsemi.no>