arm64: dts: ti: k3-am64-phycore-som: Add serial_flash label

Label the spi nor as serial_flash. This allows us to disable the
flash with an overlay common to all am6xx-phycore-som boards.

Signed-off-by: Nathan Morrisson <nmorrisson@phytec.com>
Link: https://lore.kernel.org/r/20240613230759.1984966-2-nmorrisson@phytec.com


Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>