board: mediatek: Add pumpkin board support

The pumpkin board is made by Gossamer Engineering and is using
a MediaTek SoC. The board currently comes in two available version:
MT8516 SoC and MT8167 SoC.
The board provides the following IOs: eMMC, NAND, SD card, USB type-A,
Ethernet, Wi-Fi, Bluetooth, Audio (jack out, 2 PDM port, 1 analog in),
serial over USB, and an expansion header.

Additionally there is a HDMI port, DSI port, and camera port only
on the MT8167 version of the board.

The board can be powered by battery and/or via a USB Type-C port and
is using a PMIC MT6392.

The eMMC and NAND are sharing pins and cannot be used together.

This commit is adding the basic boot support for the Pumpkin MT8516
board on the eMMC.

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