soc: keystone_serdes: create a separate SGMII SerDes driver

This patch split the Keystone II SGMII SerDes related code from
Ethernet driver and create a separate SGMII SerDes driver.
The SerDes driver can be used by others keystone subsystems
like PCI, sRIO, so move it to driver/soc/keystone directory.

Add soc specific drivers directory like in the Linux kernel.
It is going to be used by keysotone soc specific drivers.

Signed-off-by: Hao Zhang <hzhang@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>