net: stmmac: Enable SERDES power up/down sequence

This patch is to enable Intel SERDES power up/down sequence. The SERDES
converts 8/10 bits data to SGMII signal. Below is an example of
HW configuration for SGMII mode. The SERDES is located in the PHY IF
in the diagram below.

<-----------------GBE Controller---------->|<--External PHY chip-->
+----------+         +----+            +---+           +----------+
|   EQoS   | <-GMII->| DW | < ------ > |PHY| <-SGMII-> | External |
|   MAC    |         |xPCS|            |IF |           | PHY      |
+----------+         +----+            +---+           +----------+
       ^               ^                 ^                ^
       |               |                 |                |
       +---------------------MDIO-------------------------+

PHY IF configuration and status registers are accessible through
mdio address 0x15 which is defined as mdio_adhoc_addr. During D0,
The driver will need to power up PHY IF by changing the power state
to P0. Likewise, for D3, the driver sets PHY IF power state to P3.

Signed-off-by: Voon Weifeng <weifeng.voon@intel.com>
Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>