spi: rb4xx: add corresponding device tree documentation

This patch adds the correcsponding MikroTik vendor and device tree
documentation

Signed-off-by: Christopher Hill <ch6574@gmail.com>
Link: https://lore.kernel.org/r/20200521183631.37806-3-ch6574@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>
2 files changed