uboot-sunxi: rename Sinovoip BPI M2 Plus to Bananapi M2 Plus H3

uboot-sunxi: rename Sinovoip BPI M2 Plus to Bananapi M2 Plus H3

Rename the board as done in upstream commit 268ae6548779 ("sunxi: Rename
Sinovoip BPI M2 Plus to Bananapi M2 Plus H3") which backs the rename
with the following reasoning:

 The brand Sinovoip is used for Sinovoip's original VOIP products, while
 the Bananapi brand is for the single board computers they produce.  This
 has been verified by Bananapi. Rename the board from "Sinovoip BPI M2
 Plus" to "Bananapi M2 Plus". For the defconfig file, all lowercase is
 used.

 To support the H5 variant of this board, the "H3" suffix is added to
 the defconfig name.

This has to be done in order to allow building U-Boot past v2019.04
release where this change was introduced.

Ref: https://github.com/openwrt/openwrt/pull/2849#discussion_r396401489
Suggested-by: Pawel Dembicki <paweldembicki@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>