of_serial: Add UPF_FIXED_TYPE flag

This patch adds the UPF_FIXED_TYPE flag which will bypass the
8250's autoconfig probe for uart type. The uart type identified
by the of_serial's parse of the flat device tree will be utilized
as defined.

Signed-off-by: Dave Mitchell <dmitchell@amcc.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>