ARM: dts: am437x-gp-evm: Add eMMC support

Add eMMC pinmux and mmc2 related bits. We keep the mmc2
controller disabled as it conflits with gpmc/NAND.

To enable emmc, simply set mmc2 controller node to "okay"
and set the gpmc node to "disabled" and change the
SelEMMCorNAND gpio-hog to output-high.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>