ARM: dts: am335x-phycore-som: Enable gpmc node in dts files

As nand flash is not always the primary storage device any more
disable it in the phycore-som include file. Enable it the device tree
files where it is populated.

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