net: macb: Fix compilation on systems without COMMON_CLK, v2

[Palmer Dabbelt]

Our patch to add support for the FU540-C000 broke compilation on at
least powerpc allyesconfig, which was found as part of the linux-next
build regression tests.  This must have somehow slipped through the
cracks, as the patch has been reverted in linux-next for a while now.
This patch applies on top of the offending commit, which is the only one
I've even tried it on as I'm not sure how this subsystem makes it to
Linus.

This patch set fixes the issue by adding a dependency of COMMON_CLK to
the MACB Kconfig entry, which avoids the build failure by disabling MACB
on systems where it wouldn't compile.  All known users of MACB have
COMMON_CLK, so this shouldn't cause any issues.  This is a significantly
simpler approach than disabling just the FU540-C000 support.

I've also included a second patch to indicate this is a driver for a
Cadence device that was originally written by an engineer at Atmel.  The
only relation is that I stumbled across it when writing the first patch.

Changes since v1 <20190624061603.1704-1-palmer@xxxxxxxxxx>:

* Disable MACB on systems without COMMON_CLK, instead of just disabling
  the FU540-C000 support on these systems.
* Update the commit message to reflect the driver was written by Atmel.