ARM: dts: omap3-devkit8000: fix NAND memory binding

Commit d36b4cd4 'ARM: OMAP2+: Add additional GPMC timing parameters'
updated GPMC binding, but omap3-devkit8000 was not updated accordingly,
resulting in a broken configuration.

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Grant Likely <grant.likely@linaro.org>