net: ethernet: ti: icss-iep: Rename icssg_iep to icss_iep

IEP is present in ICSSG on AM65x and ICSSM on AM57xx and AM335x
platforms. To make the existing driver generic, rename icssg_iep to
icss_iep. Also prefix all function names with icss to be consistent.

Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>