clk: Export clk_bulk_prepare()

Allow clk_bulk_prepare() to be referenced by kernel modules by adding
the missing EXPORT_SYMBOL_GPL().

Fixes: 266e4e9d

 ("clk: add clk_bulk_get accessories")
Reported-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>