The CLKREF clocks are all fed by the clock signal on the CXO2 pad on the
SoC. Update the definition of these clocks to allow this to be wired up
to the appropriate clock source.

Retain "xo" as the global named parent to make the change a nop in the
event that DT doesn't carry the necessary clocks definition.
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lkml.kernel.org/r/20200106080546.3192125-2-bjorn.andersson@linaro.orgSigned-off-by: Stephen Boyd <sboyd@kernel.org>