mtd: rawnand: Deprecate ->erase()

The ->erase() hook have been overloaded by some drivers for bad reasons:
either the driver was not fitting in the NAND framework and should have
been an MTD driver (docg4), or the driver uses a specific path for the
ERASE operation (denali), instead of implementing it generically.
In any case, we should discourage people from overloading this method
and encourage them to implement ->exec_op() instead.

Move the ->erase() hook to the nand_legacy struct to make it clear.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>