mtd: nand: gpio: improve comments about GPIO assisted operation

The drivers/mtd/nand/gpio.c driver does not GPIO bitbang the complete
NAND protocol, but instead is GPIO _assisted_ -- a memory mapped interface
communicates commands and data, and only few control signals are connected
to GPIO pins.

Expand comments in the driver source and in the Kconfig description to
better reflect the very nature of the driver.  The previous text could be
mistaken for complete GPIO bitbanging.

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