ARM: dts: dra7-evm: Fix comment about NAND configuration

The switch configuration for NAND is actually the other way round.
Also mention ON/OFF states as that is more natural to understand
(without the help of schematics) when compared to HIGH/LOW.

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