spl: nand: sunxi: introduce the nand_wait_int() helper

The pattern of polling on a status register until a bit is set or a
timeout occurs is repeated multiple times in the driver. Mutualize the
code by introducing the nand_wait_int() helper that does wait for the
bit to flip or returns an error in case of timeout.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>