riscv: Add SiPeed MAIX BiT board device tree

Add the device tree sipeed_maix_bit.dts for the SiPeed MAIX BiT and
MAIX BiTm boards. This device tree enables 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>