arm: apple: Switch to fully dynamic mem layout

Support for Apple M1 Pro and Max will allow using a single binary for
all M1 SoCs. The M1 Pro/Max have a different memory layout. The RAM
start address is 0x100_0000_0000 instead of 0x8_0000_0000.
Replace the hardcoded memory layout with dynamic initialized
environment variables in board_late_init().

Tested on Mac Mini (2020) and Macbook Pro 14-inch (2021).

Signed-off-by: Janne Grunau <j@jannau.net>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>