serial: sh-sci: Set the UPF_FIXED_PORT flag

The base address, IRQ and baud rate generator parent clock rate can't be
changed by userspace. Mark the port as fixed.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>