riscv: Add device tree for K210 and Sipeed Maix BitM

Where possible, I have tried to find compatible drivers based on the layout
of registers. However, many devices remain untested. All untested devices
have been left disabled, but some tentative properties (such as compatible
strings, and clocks, interrupts, and resets properties) have been added.

Signed-off-by: Sean Anderson <seanga2@gmail.com>