phy: ti: introduce phy-gmii-sel driver

TI am335x/am437x/dra7(am5)/dm814x CPSW3G Ethernet Subsystem supports two
10/100/1000 Ethernet ports with selectable G/MII, RMII, and RGMII
interfaces. The interface mode is selected by configuring the MII mode
selection register(s) (GMII_SEL) in the System Control Module chapter
(SCM). GMII_SEL register(s) and bit fields placement in SCM are different
between SoCs while fields meaning is the same.

Historically CPSW external Port's interface mode selection configuration
was introduced using custom API and driver cpsw-phy-sel.c. This leads to
unnecessary driver, DT binding and custom API support effort.

This patch introduces CPSW Port's PHY Interface Mode selection Driver
(phy-gmii-sel) which implements standard Linux PHY interface and used
as a replacement for TI's specific driver cpsw-phy-sel.c and corresponding
custom API.

Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>