arch: kconfig: Set flash size/address to 0 by default when !XIP

Many boards/SoCs in-tree do this:
    if !XIP
    config FLASH_SIZE
        default 0
    config FLASH_BASE_ADDRESS
        default 0
    endif

And many other boards are missing this configuration (e.g. stm32 series).
Making this the default helps get non-XIP just working

Signed-off-by: Grant Ramsay <gramsay@enphaseenergy.com>