ARM: dts: am437x-gp-evm: Add gpio-hog for configuring eMMC/NAND driver

On this board either eMMC or NAND can work based on the level of
spi2_cs0.gpio0_23. Add a gpio-hog to enable configuration of this
pin in the device tree.

Move pinmux for spi2_cs0 (SEL_eMMCorNANDn) out of
NAND node into gpio0 so it is initialized with gpio0.

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