[nrf noup] boards/nordic/nrf54l15dk: Set ROM offset by PM

While Partition Manager is enabled, it manages the offset
by itself.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>

boards/nordic/nrf54l15dk/Kconfig.defconfig

@@ -7,6 +7,7 @@ config BT_CTLR
 	default BT
 
 config ROM_START_OFFSET
+	default 0 if PARTITION_MANAGER_ENABLED
 	default 0x800 if BOOTLOADER_MCUBOOT
 
 endif # BOARD_NRF54L15DK_NRF54L15_CPUAPP