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

Support u-boot driver model. We still retain
support legacy way of doing things if ELM_BASE
is defined in <asm/arch/hardware.h>

We could completely get rid of that if all
platforms defining ELM_BASE get rid of that definition
and enable CONFIG_SYS_NAND_SELF_INIT and are verified
to work.

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