net: pcs: xpcs: support to switch mode for Wangxun NICs

According to chapter 6 of DesignWare Cores Ethernet PCS (version 3.20a)
and custom design manual, add a configuration flow for switching interface
mode.

If the interface changes, the following setting is required:
1. wait VR_XS_PCS_DIG_STS bit(4, 2) [PSEQ_STATE] = 100b (Power-Good)
2. write SR_XS_PCS_CTRL2 to select various PCS type
3. write SR_PMA_CTRL1 and/or SR_XS_PCS_CTRL1 for link speed
4. program PMA registers
5. write VR_XS_PCS_DIG_CTRL1 bit(15) [VR_RST] = 1b (Vendor-Specific
   Soft Reset)
6. wait for VR_XS_PCS_DIG_CTRL1 bit(15) [VR_RST] to get cleared

Only 10GBASE-R/SGMII/1000BASE-X modes are planned for the current Wangxun
devices. And there is a quirk for Wangxun devices to switch mode although
the interface in phylink state has not changed, since PCS will change to
default 10GBASE-R when the ethernet driver(txgbe) do LAN reset.

Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>