ARM: dts: imx6q-dhcom: Set minimum memory size of all DHCOM i.MX6 variants

The minimum available memory size of all DHCOM i.MX6 variants is 512 MB.
Set this value for the memory node. If U-Boot fails to fill the memory
size, at least all DHCOM i.MX6 variants should run without problems.

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: kernel@dh-electronics.com
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Shawn Guo <shawnguo@kernel.org>