With DT enabled the following was logged:

   macb fffbc000.ethernet: no macb_clk
   macb fffbc000.ethernet: probe failed: Invalid argument

The clock probed was with the name used in a non-DT setup.
Fix so we use a proper name ("pclk") with DT, and keep
current fuctionality without DT.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>