bus: davinci: add support for da8xx bus master priority control

Create the driver for the da8xx master peripheral priority
configuration and implement support for writing to the three
Master Priority registers on da850 SoCs.

Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
[nsekhar@ti.com: subject line adjustment]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>