linker: Set ROM offset for MCUboot with NRF54L

Due to number of IRQs the ROM offset, that contains MCUboot,
needs to be moved to address that is aligned with 2048 byte
long vector table.
This comes from Cortex-M requirement where vector table needs
to be placed at offset equal to lowest power of two number
that will fit vector table.

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

boards/nordic/nrf54l15pdk/Kconfig.defconfig

diff --git a/boards/nordic/nrf54l15pdk/Kconfig.defconfig b/boards/nordic/nrf54l15pdk/Kconfig.defconfig
index bd9c723..1c83abb 100644
--- a/boards/nordic/nrf54l15pdk/Kconfig.defconfig
+++ b/boards/nordic/nrf54l15pdk/Kconfig.defconfig
@@ -6,4 +6,7 @@
 config BT_CTLR
 	default BT
 
+config ROM_START_OFFSET
+	default 0x800 if BOOTLOADER_MCUBOOT
+
 endif # BOARD_NRF54L15PDK_NRF54L15_CPUAPP