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

Commit 63aa945b1013 ("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
<[e-mail]> and Roger Quadros <[e-mail]>.

Fixes: 63aa945b1013 ("memory: omap-gpmc: Add Kconfig option for debug")
Reported-by: Uwe Kleine-König <[e-mail]>
Acked-by: Roger Quadros <[e-mail]>
Signed-off-by: Tony Lindgren <[e-mail]>

drivers/memory/Kconfig

···
 	  memory drives like NOR, NAND, OneNAND, SRAM.
 
 config OMAP_GPMC_DEBUG
-	bool
+	bool "Enable GPMC debug output and skip reset of GPMC during init"
 	depends on OMAP_GPMC
 	help
 	  Enables verbose debugging mostly to decode the bootloader provided
-	  timings. Enable this during development to configure devices
-	  connected to the GPMC bus.
+	  timings. To preserve the bootloader provided timings, the reset
+	  of GPMC is skipped during init. Enable this during development to
+	  configure devices connected to the GPMC bus.
+
+	  NOTE: In addition to matching the register setup with the bootloader
+	  you also need to match the GPMC FCLK frequency used by the
+	  bootloader or else the GPMC timings won't be identical with the
+	  bootloader timings.
 
 config MVEBU_DEVBUS
 	bool "Marvell EBU Device Bus Controller"