ARM: dts: imx28-tx28: Pass the memory unit name

Pass the memory unit name in order to fix the following
dtc warning with W=1:

arch/arm/boot/dts/imx28-tx28.dtb: Warning (unit_address_vs_reg): Node /memory has a reg or ranges property, but no unit name

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>