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