net: ti: icssg-prueth: Add mii helper apis and macros

Add MII helper APIs and MACROs. These APIs and MACROs will be later used
by ICSSG Ethernet driver. Also introduce icssg_prueth.h which has
definition of prueth related structures.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: MD Danish Anwar <danishanwar@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>