clock-alpha-pll: Add support for alpha PLL without vco mode bits

Support new type of alpha PLL that does not have vco mode bits. In
addition, add new clk_ops for alpha PLL that has pre-configured
rates.

Change-Id: Id8b80df0da43afb078d1bcbda8e1632ddc652cc8
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>