ARM: bcm283x: Define configs for RaspberryPi 4

Define two target configs for Raspberry Pi 4 (32 and 64bit) and the
corresponding BCM2838* configs.

Be aware of the current limitation in firmware which requires an
explicit configuration to force the arm in 64bit mode when the
respective target is used.

Signed-off-by: Andrei Gherzan <andrei@balena.io>
[mb: rename BCM2838 -> BCM2711]
Signed-off-by: Matthias Brugger <mbrugger@suse.com>