ARM: dts: am335x-phycore-som: Update compatible string for spi nor

As we may have different SPI NOR flashes with 8MB populated on
phyCORE-AM335x SOM, set the "jedec,spi-nor" SPI Flash compatible string.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>