clock-mmss-8994: Use alpha_pll_clk to implement MMPLL

In previous implementation, we use configure_sr_hpm_lp_pll() function to
setup the initial register values for MMPLLs. That function is designed
for earlier PLLs and its usage should be avoided.

New implementation relies on alpha_pll_clk's handoff function to do
proper register initialization during boot.

In addition, remove unused update_mask.

Change-Id: I6ebaded5898824cf221d953512a262abfa19b127
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>