bsps/riscv: Add bsp_fdt_map_intr()

This function is required by libbsd.

bsps/riscv/riscv/start/bspstart.c

@@ -194,6 +194,12 @@ uint32_t riscv_get_core_frequency(void)
   return riscv_core_freq;
 }
 
+uint32_t bsp_fdt_map_intr(const uint32_t *intr, size_t icells)
+{
+  (void) icells;
+  return intr[0];
+}
+
 void bsp_start(void)
 {
   riscv_find_harts();