kendryte/k210: Add device tree support

The Kendryte k210 board initial loader does not provide a device tree
for the firmware and subsequent payload. Add the k210.dts device tree
description file to solve this. This file describes only the SoC core
components: CPUs, memory and PLIC.

Automatically compile and add this file to the firmware generated
using the FW_PAYLOAD_FDT_PATH configuration parameter.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>