clock-alpha-pll: Add offset support for alpha_pll_clk

Each clock driver ioremaps its entire address space. Each clock inside the
controller access its registers with base + offset. Add offset support for
alpha_pll_clk so that it can be instantiated same way as all other clocks.

Change-Id: I14740b77b9f45659bbd0ba2dd421728a8426c6f9
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>