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>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Link: https://lore.kernel.org/all/20221220102203.52398-9-rogerq@kernel.org
Link: https://lore.kernel.org/all/CABGWkvrvKiVA_yaDnHJcHEKwc+pEuLdz=i6HQEY0oJQvohCUsw@mail.gmail.com