net: dsa: mt7530: Add the support of MT7531 switch

Add new support for MT7531:

MT7531 is the next generation of MT7530. It is also a 7-ports switch with
5 giga embedded phys, 2 cpu ports, and the same MAC logic of MT7530. Cpu
port 6 only supports SGMII interface. Cpu port 5 supports either RGMII
or SGMII in different HW sku. Due to SGMII interface support, pll, and
pad setting are different from MT7530. This patch adds different initial
setting, and SGMII phylink handlers of MT7531.

MT7531 SGMII interface can be configured in following mode:
- 'SGMII AN mode' with in-band negotiation capability
    which is compatible with PHY_INTERFACE_MODE_SGMII.
- 'SGMII force mode' without in-bnad negotiation
    which is compatible with 10B/8B encoding of
    PHY_INTERFACE_MODE_1000BASEX with fixed full-duplex and fixed pause.
- 2.5 times faster clocked 'SGMII force mode' without in-bnad negotiation
    which is compatible with 10B/8B encoding of
    PHY_INTERFACE_MODE_2500BASEX with fixed full-duplex and fixed pause.

Signed-off-by: Landen Chao <landen.chao@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>

drivers/net/dsa/Kconfig

@@ -33,12 +33,12 @@ config NET_DSA_LANTIQ_GSWIP
 	  the xrx200 / VR9 SoC.
 
 config NET_DSA_MT7530
-	tristate "MediaTek MT7530 and MT7621 Ethernet switch support"
+	tristate "MediaTek MT753x and MT7621 Ethernet switch support"
 	depends on NET_DSA
 	select NET_DSA_TAG_MTK
 	help
-	  This enables support for the MediaTek MT7530 and MT7621 Ethernet
-	  switch chip.
+	  This enables support for the MediaTek MT7530, MT7531, and MT7621
+	  Ethernet switch chips.
 
 config NET_DSA_MV88E6060
 	tristate "Marvell 88E6060 ethernet switch chip support"