mtd: rawnand: Prepare the late addition of supported operation checks

Add an empty envelope just to show how to add additional checks for new
operations. This is going to be used for sequential cached reads, which
require the page size to be known (and the discovery to be over), hence
the "late" designation.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Liao Jaime <jaimeliao.tw@gmail.com>
Link: https://lore.kernel.org/linux-mtd/20230112093637.987838-3-miquel.raynal@bootlin.com