riscv: dts: starfive: add Milkv Mars board device tree

riscv: dts: starfive: add Milkv Mars board device tree

The Milkv Mars is a development board based on the Starfive JH7110 SoC.
The board features:

- JH7110 SoC
- 1/2/4/8 GiB LPDDR4 DRAM
- AXP15060 PMIC
- 40 pin GPIO header
- 3x USB 3.0 host port
- 1x USB 2.0 host port
- 1x M.2 E-Key
- 1x eMMC slot
- 1x MicroSD slot
- 1x QSPI Flash
- 1x 1Gbps Ethernet port
- 1x HDMI port
- 1x 2-lane DSI and 1x 4-lane DSI
- 1x 2-lane CSI

Add the devicetree file describing the currently supported features,
namely PMIC, UART, I2C, GPIO, SD card, QSPI Flash, eMMC and Ethernet.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>