Enable arm,io-coherent property of PL310 L2 cache on Armada 38x platforms

[mw_semihalf.com]

Summary

This patch disables outer cache sync in PL310 driver
by adding "arm,io-coherent" property. In addition to
the previous patches it was the last bit needed
for enabling proper operation of Armada 38x SoCs
with the IO cache coherency.

[mw_semihalf.com]

Add "arm,io-coherent" to PL310 node (see https://reviews.freebsd.org/D11245)

[mw_semihalf.com]

Do not add redundant 'dma-coherent', and add the property only for the PL310 node. Unlike linux, we do not have to do it in runtime, hacking the platform code.