board: mediatek: rename pumpkin board into mt8516

More than one pumpkin board has been made with different MediaTek SoCs.
Rename the pumpkin board to follow the naming convention of all
other MediaTek boards and also to not be confusing when other pumpkin
boards will be added in follow-up commits.

Signed-off-by: Fabien Parent <fparent@baylibre.com>