nRF52840: Set mpu-rom-end to 0x1fffffff
nRF52840: Set mpu-rom-end to 0x1fffffff

So we have at least one test platform exercising the special case of
mpu-rom-end being 0x1fffffff, set that for nRF52840.

andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,kjbracey-arm/mbed