Documentation: devicetree: Remove unimac-mdio bindings from kernel

This is a preparation change for merging android-4.19-q.85 into
msm-4.19 branch.

Remove unimac-mdio bindings from kernel added by [1].

[1] is conflicting against [2] in msm-4.19.

[1] 590ee2e0 ("net: phy: mdio-bcm-unimac: Allow configuring MDIO clock divider")
[2] 728aba9a("Documentation: devicetree: Remove devicetree bindings from kernel")

Change-Id: Id2227f0133eaf1a1ce1c3ba317c69471b27e96bd
Signed-off-by: Ivaylo Georgiev <irgeorgiev@codeaurora.org>