cpu/mips32r2_generic: Add PERIPH_TIMER_PROVIDES_SET

mips32r2_common already implements timer_set so it should not be provided by
periph_common/timer to avoid multiple definition errors currently hidden by the
linker.

The firmware was using the one from `mips32r2_common` before (binary checked).
So behaviour is identical.

cpu/mips32r2_generic/include/periph_cpu.h

@@ -24,6 +24,11 @@ extern "C" {
 #define PROVIDES_PM_SET_LOWEST
 /** @} */
 
+/**
+ * @brief   Prevent shared timer functions from being used
+ */
+#define PERIPH_TIMER_PROVIDES_SET   1
+
 #ifdef __cplusplus
 }
 #endif