net: macb: Fix compilation on systems without COMMON_CLK

[Palmer Dabbelt <palmer@sifive.com>]

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 another Kconfig entry to
conditionally enable the FU540-C000 support.  It would be less code to
just make MACB depend on COMMON_CLK, but I'm not sure if that dependency
would cause trouble for anyone so I didn't do it that way.  I'm happy to
re-spin the patch to add the dependency, but as it's a very small change
I'm also happy if you do it yourself :).

I've also included a second patch to indicate this is a Cadence driver,
not an Atmel driver.  As far as I know the controller is from Cadence,
but it looks like maybe it showed up first on some Atmel chips.  I'm
fine either way, so feel free to just drop it if you think the old name
is better.  The only relation is that I stumbled across it when writing the
first patch.