ARM: dts: imx6: Pass memory unit-adress

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

Warning (unit_address_vs_reg): Node /memory has a reg or ranges property, but no unit name

There are cases where dts passes an empty memory node, 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>
Acked-By: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>