clk: qcom: Add support to export clk_rcg2_calc_rate()

There could be some use cases where clk_rcg2_calc_rate()
needs to be use in other modules for calculating rate, so
add support for the same by exporting it as kernel symbol.

Change-Id: If0840f3f2759d97c99bf3d06e3caf91d9a0ede73
Signed-off-by: Amit Nischal <anischal@codeaurora.org>