dm: mmc: sunxi: Add A10/A20 compatible strings

Commit dd27918c

 ("dm: mmc: sunxi: Add support for driver model")
only added the allwinner,sun5i-a13-mmc compatible string for this
driver. The DM initialisation code here also works with (at least) A10
and A20, so add the appropriate compatible strings as per Linux 4.17's
driver.

Tested on A10 Cubieboard and A20 pcDuino3 Nano with CONFIG_DM_MMC.
(A20 worked already, because sun7i-a20.dtsi specifies both the A13 and
A20 strings.)

Signed-off-by: Adam Sampson <ats@offog.org>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>