arm64: dts: phycore-imx8mm: fix nor mtd partitions

Add #address-cells and #size-cells properties to som_flash. This
enables correct parsing of added SPI NOR mtd partition child nodes that
are set in the bootloader.

Signed-off-by: Yannic Moog <y.moog@phytec.de>
Signed-off-by: Cem Tenruh <c.tenruh@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>