mtd: rawnand: brcmnand: legacy exec_op implementation

Commit 3c8260ce ("mtd: rawnand: brcmnand: exec_op implementation")
removed legacy interface functions, breaking < v5.0 controllers support.
In order to fix older controllers we need to add an alternative exec_op
implementation which doesn't rely on low level registers.

Fixes: 3c8260ce ("mtd: rawnand: brcmnand: exec_op implementation")
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: David Regan <dregan@broadcom.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: William Zhang <william.zhang@broadcom.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>