clk: clock-alpha-pll: Support parsing alpha-pll clocks from dt

Alpha PLL clocks are a type of PLLs which provide an output rate of
output = input * ( L + a * 2 ^-40).
Allow defining this clock type in dt.

Change-Id: I062e502d9217332bca4e7df6fa6b8c62ac313d23
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>