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>
diff --git a/configs/malta_defconfig b/configs/malta_defconfig
index 2b43818..0680f59 100644
--- a/configs/malta_defconfig
+++ b/configs/malta_defconfig
@@ -26,6 +26,7 @@
 CONFIG_FLASH_CFI_DRIVER=y
 CONFIG_SYS_FLASH_USE_BUFFER_WRITE=y
 CONFIG_SYS_FLASH_CFI=y
+CONFIG_PCNET=y
 CONFIG_PCI=y
 CONFIG_RTC_MC146818=y
 CONFIG_SYS_NS16550=y