ARM: dts: imx5: Pass the memory unit-address

Pass the memory unit-address to fix the following build warnings with W=1:

arch/arm/boot/dts/imx51-zii-rdu1.dtb: Warning (unit_address_vs_reg): Node /memory has a reg or ranges property, but no unit name
arch/arm/boot/dts/imx53-tx53-x03x.dtb: Warning (unit_address_vs_reg): Node /memory has a reg or ranges property, but no unit name
arch/arm/boot/dts/imx53-tx53-x13x.dtb: Warning (unit_address_vs_reg): Node /memory has a reg or ranges property, but no unit name

In theses cases an empty memory node is passed, which will be filled
by the bootloader. Passing the memory base address still allows the
bootloader to fill the memory size.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>