riscv: Add SiPeed MAIXDUINO board device tree

Add the device tree sipeed_maixduino.dts for the SiPeed MAIXDUINO board.
This device tree enables LEDs and spi/mmc SD card device. Additionally,
gpios and i2c are also enabled and mapped to the board header pins as
indicated on the board itself.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
[Palmer: Remove undocumented microphone entry, along with the use.]
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>