net: ti: icssg-prueth: Add Firmware Interface for ICSSG Ethernet driver.

Add firmware interface related headers and macros for ICSSG Ethernet
driver. These macros will be later used by the ICSSG ethernet driver.

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>