interconnect: Add of_icc_get_by_index() helper function

This is the same as the traditional of_icc_get() function, but the
difference is that it takes index as an argument, instead of name.

Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Sibi Sankar <sibis@codeaurora.org>
Link: https://lore.kernel.org/r/20200512125327.1868-4-georgi.djakov@linaro.org


Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>