ARM: dts: artpec: use 1 GiB RAM

There has never been an artpec6 devboard produced
with less than 1 GiB RAM.

Increase the default value to 1 GiB RAM, so that we
can netboot with large initramfs without going OOM.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>