riscv: Add SiPeed MAIX GO board device tree

Add the device tree sipeed_maix_go.dts for the SiPeed MAIX GO board.
This device tree enables buttons, LEDs, gpio, i2c and spi/mmc SD card
devices.

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>