boards: nrf54h20dk: Limit app core FLASH_LOAD_SIZE

CONFIG_USE_DT_CODE_PARTITION had to be disabled to add MCUboot support.
As a result, CONFIG_FLASH_LOAD_SIZE was left at zero, which means that
the linker would claim all available MRAM for the app core.

For now, we can't allow that, because the default nRF54H20 DK memory map
divides MRAM between multiple cores in order to support various samples.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>