mtd: nand: sunxi: Convert to the driver model

mtd: nand: sunxi: Convert to the driver model

Clocks, resets, and pinmuxes are now handled by the driver model, so the
only thing the "board" code needs to do is load the driver. This matches
the pattern used by other DM raw NAND drivers (there is no NAND uclass).

The actual board code is now only needed in SPL.

Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>