Enable pl310 coherent operation in platform init for Armada 38x

Updating PL310 sotfware context sc_io_coherent field in
platform_pl310_init() routine for Armada 38x helps to avoid
using 'arm,io-coherent' property, which is by default not present
in the device tree node in Linux.

This way another step for DT unification between two operating
systems is done. The improvemnt will also work after enabling
PLATFORM for Marvell ARMv7 SoCs.

Reviewed by: andrew, cognet (mentor)
Approved by: cognet (mentor)
Obtained from: Semihalf
Differential Revision: https://reviews.freebsd.org/D11883