dt-bindings: net: Add IEP property in ICSSG

Add IEP property in ICSSG hardware DT binding document.
ICSSG uses IEP (Industrial Ethernet Peripheral) to support timestamping
of ethernet packets, PTP and PPS.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: MD Danish Anwar <danishanwar@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>