mtd: rawnand: omap_gpmc: support u-boot driver model

Adds driver model support.

We need to be able to self initialize the NAND controller/chip
at probe and so enable CONFIG_SYS_NAND_SELF_INIT.

Doing so requires nand_register() API which is provided by nand.c
and needs to be enabled during SPL build via CONFIG_SPL_NAND_INIT.
But nand.c also provides nand_init() so we need to get rid of nand_init()
in omap_gpmc driver if CONFIG_SPL_NAND_INIT is set.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>