ARM: dts: imx6ul-phytec-phycore-som: Add missing unit name

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

arch/arm/boot/dts/imx6ul-phytec-phycore-som.dtsi:23.9-26.4: Warning (unit_address_vs_reg): /memory: node has a reg or ranges property, but no unit name

Cc: Stefan Riedmueller <s.riedmueller@phytec.de>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>