[nrf noup] boards: nordic: nrf54lm20 adjust ROM_START_OFFSET

adds kconfig that was probably skipped during migration.

Signed-off-by: Mateusz Michalek <[email protected]>
(cherry picked from commit cd135b2)

Author: michalek-no

boards/nordic/nrf54lm20dk/Kconfig.defconfig

@@ -7,6 +7,7 @@ config HW_STACK_PROTECTION
 	default ARCH_HAS_STACK_PROTECTION
 
 config ROM_START_OFFSET
+	default 0 if PARTITION_MANAGER_ENABLED
 	default 0x800 if BOOTLOADER_MCUBOOT
 
 endif # BOARD_NRF54LM20DK_NRF54LM20A_CPUAPP