clk: msm: mdmfermium: Change parent entry for apss_ahb_clk_src

Make apss_ahb_clk_src derive 19.2MHz from xo_a instead of xo. This will
let xo shutdown to happen.

Change-Id: I0e1eec998f0df56715ad1fb936f58c7d0bb3bcba
Signed-off-by: Jaydeep Sen <jsen@codeaurora.org>