net: ti: am65-cpsw-nuss: Prepare to support non primary ext port

CPSW NUSS IP on K3 SoCs can have more than one external port (upto 8)
Therefore increase AM65_CPSW_CPSWNU_MAX_PORTS to 9 (8 ext + 1 Root port)
as preparation to allow any one of the 8 ports to be used as ethernet
interface in U-Boot.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>