sunxi: rename Bananapi M3 dts file name

The upstream (Linux) device tree file for the Bananapi M3 follows the
convention of using the well known brand name, instead of the vendor
name, for naming. The file was recently added to upstream in commit
359b5a1e1c2d ("ARM: sun8i: a83t: Add device tree for Sinovoip Bananapi
BPI-M3")

Rename the device tree file in U-boot to match.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>