meson-gx: reserved memory regions

The Odroid C2 has two GiB of memory with two reserved regions.
reg = <0x0 0x0 0x0 0x1000000>;
reg = <0x0 0x10000000 0x0 0x200000>;

Patch
bfcef28a (arm: add initial support for Amlogic Meson and
ODROID-C2) provided function dram_init_banksize to reserve the
first 16 MiB of RAM for firmware in function dram_init_banksize
in arch/arm/mach-meson/board.c and defined
CONFIG_NR_DRAM_BANKS = 1.

With this patch dram_init_banksize is changed to additionally
reserve the 2MiB region for the ARM Trusted Firmware (BL31).
CONFIG_NR_DRAM_BANKS is set to 2.

Cc: Andreas Färber <afaerber@suse.de>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>