clk: qcom: handle alpha PLLs with 16bit alpha val registers

Some alpha PLLs have support for only a 16bit programable Alpha Value
(as against the default 40bits). Add a flag to handle the 16bit alpha
registers

Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>