drivers/net/ethernet: add some basic kdoc tags

A couple of drivers had a "generic documentation" section that
would trigger a "can't understand" message from W=1 compiles.

Fix by using correct DOC: tags in the generic sections.

Fixed Warnings:
drivers/net/ethernet/arc/emac_arc.c:4: info: Scanning doc for c
drivers/net/ethernet/cadence/macb_pci.c:3: warning: missing initial short description on line:
 * Cadence GEM PCI wrapper.
drivers/net/ethernet/cadence/macb_pci.c:3: info: Scanning doc for Cadence
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/arc/emac_arc.c

 // SPDX-License-Identifier: GPL-2.0-or-later
 /**
- * emac_arc.c - ARC EMAC specific glue layer
+ * DOC: emac_arc.c - ARC EMAC specific glue layer
  *
  * Copyright (C) 2014 Romain Perier
  *

drivers/net/ethernet/cadence/macb_pci.c

 // SPDX-License-Identifier: GPL-2.0-only
 /**
- * Cadence GEM PCI wrapper.
+ * DOC: Cadence GEM PCI wrapper.
  *
  * Copyright (C) 2016 Cadence Design Systems - https://www.cadence.com
  *