drivers/opp: Remove "select SRCU"

Now that the SRCU Kconfig option is unconditionally selected, there is
no longer any point in selecting it.  Therefore, remove the "select SRCU"
Kconfig statements.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Viresh Kumar <vireshk@kernel.org>
Cc: Nishanth Menon <nm@ti.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: <linux-pm@vger.kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

drivers/opp/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
 config PM_OPP
 	bool
-	select SRCU
 	help
 	  SOCs have a standard set of tuples consisting of frequency and
 	  voltage pairs that the device will support per voltage domain. This