ARM: dts: am335x-phycore-som: Add emmc node

The phyCORE-AM335x EMMC modules have an EMMC populated and are based
on the phyCORE-AM335x R2 SOM. Therefore, the EMMC node will be added to
the phycore-som devce tree but is by default disabled.

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