serial-uartlite: Enlarge maximum nr of devices to 16.

This device is ideal to use when you need a lot of uarts in your FPGA.
Try not to force all those users to patch their kernel.

Signed-off-by: Maarten Brock <m.brock@vanmierlo.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>