drivers: net: cpsw-phy-sel: Add new driver for phy mode selection for cpsw

The cpsw currently lacks code to properly set up the hardware interface
mode on AM33xx. Other platforms might be equally affected.

Usually, the bootloader will configure the control module register, so
probably that's why such support wasn't needed in the past. In suspend
mode though, this register is modified, and so it needs reprogramming
after resume.

This patch adds a new driver in which hardware interface can configure
correct register bits when the slave is opened.

The AM33xx also has a bit for each slave to configure the RMII reference
clock direction. Setting it is now supported by a per-slave DT property.

This code path introducted by this patch is currently exclusive for
am33xx and same can be extened to various platforms via the DT compatibility
property.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Tested-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>