riscv: Add Kendryte K210 device tree

Add a generic device tree for Kendryte K210 SoC based boards. This is
for now a very simple device tree describing the core elements of the
SoC. This is suitable (and tested) for the Kendryte KD233 development
board, the Sipeed MAIX M1 Dan Dock board and the Sipeed MAIXDUINO board.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>