RPI: Add defconfigs for rpi4 (32/64)

RPI: Add defconfigs for rpi4 (32/64)

This defines a minimum defconfig for each of the two Raspberry Pi 4
variants. One notable difference is that we don't have a embedded dt for
this board given that the fw supplies us with one which we can reuse.
Furthermore, the ram size is not queryable through mbox interface as the
maximum reported size is 1G. The fw patches the dt with the right
memory configuration and uboot uses it as it is. We avoid u-boot
touching this configuration by making sure CONFIG_ARCH_FIXUP_FDT_MEMORY
is deactivated.

Signed-off-by: Andrei Gherzan <andrei@balena.io>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>