mtd: rawnand: omap_gpmc: Add SPL NAND support

Enables SPL NAND support for ARCH_K3 by enabling
SPL_NAND_INIT and SPL_SYS_NAND_SELF_INIT.

Legacy OMAP2plus platforms still rely on SPL_NAND_AM33XX_BCH
instead.

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