ARM: dts: dra7: Set a default parent to mcasp3_ahclkx_mux

Assign a default parent to mcasp3_ahclkx_mux clock using
the assigned-clock-parents property. This is helpful in
cases like kexec where in the clock parent can be something
other than the value at reset.

Suggested-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>