opp: Add support for parsing interconnect bandwidth

The OPP bindings now support bandwidth values, so add support to parse it
from device tree and store it into the new dev_pm_opp_icc_bw struct, which
is part of the dev_pm_opp.

Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
[ Viresh: Create _read_bw() and use it, renamed _of_find_icc_paths() to
	  dev_pm_opp_of_find_icc_paths(), exported it and made opp_table
	  argument optional. Also drop the depends on from Kconfig. ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>