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: https://github.com/bbrezillon/linux/commit/4ec6f8d8d83f5aaca5d1877f02d48da96d41fcba
Link: https://github.com/bbrezillon/linux/commit/11b4acffd761c4928652d7028d19fcd6f45e4696Signed-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