dt-bindings: slave-device: add current-speed property

This adds a new DT property to define the current baud rate of the
slave device.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>