net: mediatek: split ethernet switch code from mtk_eth.c

mtk_eth.c contains not only the ethernet GMAC/DMA driver, but also
some ethernet switch initialization code. As we may add more switch
support in the future, it's better to move them out of mtk_eth.c to
avoid increasing the code complexity.

Since not all switches are supported for a particular board, Kconfig
options are added to allow user to select which switch should be
built into u-boot. If multiple switches are selected, auto-detecting
can also be enabled.

diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
index 88ff025..e48e28d 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -966,26 +966,7 @@
 	  This driver implements support for the (Enhanced) Three-Speed
 	  Ethernet Controller found on Freescale SoCs.
 
-config MEDIATEK_ETH
-	bool "MediaTek Ethernet GMAC Driver"
-	select PHYLIB
-	select DM_GPIO
-	select DM_RESET
-	help
-	  This Driver support MediaTek Ethernet GMAC
-	  Say Y to enable support for the MediaTek Ethernet GMAC.
-
-if MEDIATEK_ETH
-
-config MTK_ETH_SGMII
-	bool
-	default y if ARCH_MEDIATEK && !TARGET_MT7623
-
-config MTK_ETH_XGMII
-	bool
-	default y if TARGET_MT7987 || TARGET_MT7988
-
-endif # MEDIATEK_ETH
+source "drivers/net/mtk_eth/Kconfig"
 
 config HIFEMAC_ETH
 	bool "HiSilicon Fast Ethernet Controller"