ARM: meson-g12a: Handle 4GiB DRAM size

When configured with 4GiB DRAM size, only 3.8GiB is available, the
I/O beeing mapped in the last 256MiB of the first 4GiB physical memory/

First fixup the mm_region to handle the first 3.8GiB as memory and the
last 256MiB as I/O.

Then limit the real memory reported by the firmware to the available
physical space, 3.8GiB aligned with the mm_region memory zone size.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Mark Kettenis <kettenis@openbsd.org>