ARM: OMAP2+: Convert NAND to retrieve GPMC settings from DT

When booting with device-tree, retrieve GPMC settings for NAND from
the device-tree blob. This will allow us to remove all static settings
stored in the gpmc-nand.c in the future once the migration to
device-tree is complete.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>