devicetree: bindings: improve description for GPIO assisted NAND flash

Expand the description of the 'gpios' property in the GPIO assisted
NAND flash binding, to explicitly list the required GPIO pin references
and their order.

Update the example section to individually bracket the GPIO references,
and capitalize the signal names for improved readability.

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>