Change a53-pll driver to use clk_parent_data rather than always looking
up the xo clock in the system clock list.

Note, this change also switches the a53-pll from the global `xo' clock
to the `xo_board', the clock that is specified as the `xo' clock in the
DT file.
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20220909103137.3727830-1-dmitry.baryshkov@linaro.org