memory: omap-gpmc: Fix unselectable debug option for GPMC

Commit 63aa945b ("memory: omap-gpmc: Add Kconfig option for debug")
added a debug option for GPMC, but somehow managed to keep it unselectable.

This probably happened because I had some uncommitted changes and the
GPMC option is selected in the platform specific Kconfig.

Let's also update the description a bit, it does not mention that
enabling the debug option also disables the reset of GPMC controller
during the init as pointed out by Uwe Kleine-König
<u.kleine-koenig@pengutronix.de> and Roger Quadros <rogerq@ti.com>.

Fixes: 63aa945b ("memory: omap-gpmc: Add Kconfig option for debug")
Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>