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

One bit in the control registers indicates if the NAND controller is
ready to receive a new command. Otherwise, the command FIFO is full and
we should wait for this bit to flip. It then states that the last
command has been processed and the FIFO is now free to welcome another
command.

Add this sanity check before starting any new command.

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>