[MIPS] Suppress compiler warnings

The excite platform exports hardware resources for device drivers to use.
Any driver wanting to use these resources will look up them by their names.
Since these resources are declared to have static linkage, but are not
used in the source file defining them, the compiler used to emit an
'unused' warning, which this patch suppresses.

Signed-off-by: Thomas Koeller <thomas.koeller@baslerweb.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>