SERIAL: 8250: Remove SERIAL_8250_AU1X00

Remove the SERIAL_8250_AU1X00 config symbol.  Instead, use the MIPS_ALCHEMY
one which is always defined when building an Au1x00-based platform.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
To: Linux-MIPS <linux-mips@linux-mips.org>
Cc: Linux-serial <linux-serial@vger.kernel.org>
Patchwork: https://patchwork.linux-mips.org/patch/1461/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

This one depends on a previous patch (which removes SOC_AU1X00 and changes
MACH_ALCHEMY) to apply cleanly (and then actually work), so I'd love for
this to go in via the mips tree.