[dev][interrupt][plic] kick the max irqs to 256

Will need to figure out a cleaner way of doing this, but for now just
keep increasing the size.

dev/interrupt/riscv_plic/plic.c

@@ -20,9 +20,9 @@
 
 #define LOCAL_TRACE 0
 
-// Preallocate space for up to 128 vectors.
+// Preallocate space for up to 256 vectors.
 // If more are needed will need to bump this up or switch to a dynamic scheme.
-#define MAX_IRQS 128
+#define MAX_IRQS 256
 static struct int_handlers {
     int_handler handler;
     void *arg;