riscv: Add device tree bindings for SPI

This patch adds bindings for the MMC slot and SPI flash on the Sipeed Maix
Bit.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Acked-by: Rick Chen <rick@andestech.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>