cmn700: update get_port_number to account for port count

Update get_port_number to account for the number of ports in the cross
point (XP). For the XP with port count as 2, the port number is embedded
at the bit 2, while for the port count 3 and 4, the port number is
embedded in the bit fields 2:1.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I3f0baaf485357c82f5d3d612d16bc29aa41e5275