ARM: meson: Extend mem_map to support 3GiB of RAM

ARM: meson: Extend mem_map to support 3GiB of RAM

The current mem_map definition for Meson SoCs has support for up
to 2GiB of RAM. According to S905, S905X, S912 and S805X datasheets
the DDR region is set from 0x00000000 to 0xBFFFFFFF, so mem_map's
definition should be changed accordingly.

It is also needed to be able to boot Khadas VIM2 board with S912
SoC.

Signed-off-by: Loic Devulder <ldevulder@suse.de>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>