MIPS: Alchemy: fix build with SERIAL_8250=n

In commit 7d172bfe ("Alchemy: Add UART PM methods") I introduced
platform PM methods which call a function of the 8250 driver;
this patch works around link failures when the kernel is built
without 8250 support.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
To: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/1737/


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