clk: qcom: Add .is_enabled ops for clk-alpha-pll

This would be useful in subsequent patches when the .set_rate operation
would need to identify if the PLL is actually enabled

Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
[sboyd@codeaurora.org: Simplify return statement of is_enabled op]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>