powerpc: dts: t1040: add bindings for Seville Ethernet switch

Add the description of the embedded L2 switch inside the SoC dtsi file
for NXP T1040.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Maxim Kochetkov <fido_max@inbox.ru>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>