dev-arm: ambiguous use of getPort()

dev-arm: ambiguous use of getPort()

The recent introduction of getPort() creates a conflict with
the existing method used in arm MMU.

This patch rename the old getPort() in getDMAPort() according
to the returned value (DmaPort class type)

Change-Id: Ief3d83650fd6b08490522341631244be06e380ce
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17469
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>