mtd: rawnand: brcmnand: exec_op implementation

exec_op implementation for Broadcom STB, Broadband and iProc SoC
This adds exec_op and removes the legacy interface. Based on changes
proposed by Boris Brezillon.

Link: bbrezillon/linux@4ec6f8d
Link: bbrezillon/linux@11b4acf
Signed-off-by: David Regan <dregan@broadcom.com>
[Miquel Raynal: Misc style fixes]
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20231125012438.15191-4-dregan@broadcom.com

Author: David Regan