serial: vt8500: Add devicetree support for vt8500-serial

Increase vt8500_max_ports to 6 as the WM8505 as 6 available uarts.
Use devicetree port id as primary addressing for ports but allow
auto-allocation if id not specified.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Acked-by: Alan Cox <alan@linux.intel.com>