riscv: Add Sipeed Maix support

riscv: Add Sipeed Maix support

The Sipeed Maix series is a collection of boards built around the RISC-V
Kendryte K210 processor. This processor contains several peripherals to
accelerate neural network processing and other "ai" tasks. This includes a
"KPU" neural network processor, an audio processor supporting beamforming
reception, and a digital video port supporting capture and output at VGA
resolution. Other peripherals include 8M of sram (accessible with and
without caching); remappable pins, including 40 GPIOs; AES, FFT, and SHA256
accelerators; a DMA controller; and I2C, I2S, and SPI controllers. Maix
peripherals vary, but include spi flash; on-board usb-serial bridges; ports
for cameras, displays, and sd cards; and ESP32 chips. Currently, only the
Sipeed Maix Bit V2.0 (bitm) is supported, but the boards are fairly
similar.

Documentation for Maix boards is located at
<http://dl.sipeed.com/MAIX/HDK/>.  Documentation for the Kendryte K210 is
located at <https://kendryte.com/downloads/>. However, hardware details are
rather lacking, so most technical reference has been taken from the
standalone sdk located at
<https://github.com/kendryte/kendryte-standalone-sdk>.

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