clk: Add missing of_clk_set_defaults export

The of_clk_set_defaults() function is used in subsystems which can
be built as module. Add the missing symbol export entry so there
is no build errors like:
"ERROR: "of_clk_set_defaults" [drivers/i2c/i2c-core.ko] undefined!".

Fixes commit: 86be408b
"clk: Support for clock parents and rates assigned from device tree"

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Mike Turquette <mturquette@linaro.org>