net: Move CONFIG_RTL8169 to Kconfig

Introduce CONFIG_RTL8169 in Kconfig and move over boards' defconfig
to use that.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Stephen Warren <swaren@nvidia.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/configs/minnowmax_defconfig b/configs/minnowmax_defconfig
index ea39e6a..efb9c1e 100644
--- a/configs/minnowmax_defconfig
+++ b/configs/minnowmax_defconfig
@@ -29,6 +29,7 @@
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_DM_ETH=y
+CONFIG_RTL8169=y
 CONFIG_DM_PCI=y
 CONFIG_DM_RTC=y
 CONFIG_DEBUG_UART=y