clk: Add prepare lock in clk_populate_clock_opp_table

After enabling CONFIG_PROVE_LOCKING, observed dumpstacks in the
boot time.
Call trace:
    clk_core_round_rate_nolock+0xd4/0xe0
    clk_hw_round_rate+0x4c/0x80
    clk_populate_clock_opp_table+0x168/0x318
    devm_clk_hw_register+0x88/0x9c
    devm_clk_register_regmap+0x54/0x60
    qcom_cc_really_probe+0x114/0x208

clk_hw_round_rate() required to be called with prepare lock held.
So acquire prepare lock before invoking clk_hw_round_rate().

Change-Id: I608e9e7cfe7a5a43fa0e0543b909cefe59030067
Signed-off-by: Veera Vegivada <vvegivad@codeaurora.org>