interconnect: Add get_bw() callback

The interconnect controller hardware may support querying the current
bandwidth settings, so add a callback for providers to implement this
functionality if supported.

Link: https://lore.kernel.org/r/20200825170152.6434-2-georgi.djakov@linaro.org
Reviewed-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>