net: pcnet: Add Kconfig entries

Add Kconfig entries for the pcnet driver and convert MIPS malta
to use those.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>