arm: apple: Add initial support for Apple's M1 SoC

Add support for Apple's M1 SoC that is used in "Apple Silicon"
Macs.  This builds a basic U-Boot that can be used as a payload
for the m1n1 boot loader being developed by the Asahi Linux
project.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
[trini: Add MAINTAINERS entry]