This enables configs necessary for using SPI. The environment is saved to
the very end of SPI flash. This is unlikely to be overwritten unless the
entire flash is reprogrammed.

This also supplies a default bootcommand. It loads an image and device tree
from the first partition of the MMC. This is a minimal/least effort
bootcmd, so suggestions (especially in the form of patches) are welcome. I
didn't set up distro boot because I think it is unlikely that any
general-purpose linux distros will ever be ported to this board.
Signed-off-by: NSean Anderson <seanga2@gmail.com>
Tested-by: NChrstopher Obbard <obbardc@gmail.com>
Reviewed-by: NRick Chen <rick@andestech.com>

This patch adds the necessary configs and docs for FPIOA and GPIO support
on the K210.
Signed-off-by: NSean Anderson <seanga2@gmail.com>

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: NSean Anderson <seanga2@gmail.com>