rpi_4_defconfig: add missing CONFIG_ARCH_FIXUP_FDT_MEMORY

As discussed at https://lore.kernel.org/linux-arm-kernel/b726290c-1038-3771-5187-6ac370bc92c9@arm.com/T/


the defconfig for rpi4 miss CONFIG_ARCH_FIXUP_FDT_MEMORY.
Without it, booting with an initrd fail.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>