clk: msm: clock-osm: Set the HMSS GPLL0 RCG to run at 300MHz

The default settings of the gcc_hmss_gpll0_clk_src make it
run at 600 MHz. Call set rate on the clock so that its
divider settings can be programmed.

CRs-Fixed: 989118
Change-Id: I49aee860dd3f0f4f7ecb024228f182d126424906
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>