clk: core: Avoid double initialization of clocks

Some clock providers can be initialized via of_clk_init() and also via
platform device probe.

Avoid double initialization of them by setting the OF_POPULATED flag.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>