spl: nand: sunxi: create an helper to handle command execution

Executing a command is matter of always doing the following sequence:
  * Waiting for the FIFO to be empty so we can fill it with the new
    command.
  * Clearing the status register.
  * Writing the command in the FIFO.
  * Waiting for the command to finish.

Add a nand_exec_cmd() helper to handle this instead of repeating the
logic through the various functions.

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>