[MIPS] feature-removal-schedule.txt: Schedule au1x00_uart for removal.

The 8250 serial driver now has the ability to deal with the differences
between the standard 8250 family of UARTs and their slightly strange
brother on Alchemy SOCs.  The loss of features is not considered an
issue.
 
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>