Make CPURead/WriteFunc structure 'const'

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/hw/pl080.c b/hw/pl080.c
index 9c17be6..a6ea8eb 100644
--- a/hw/pl080.c
+++ b/hw/pl080.c
@@ -307,13 +307,13 @@
     pl080_update(s);
 }
 
-static CPUReadMemoryFunc *pl080_readfn[] = {
+static CPUReadMemoryFunc * const pl080_readfn[] = {
    pl080_read,
    pl080_read,
    pl080_read
 };
 
-static CPUWriteMemoryFunc *pl080_writefn[] = {
+static CPUWriteMemoryFunc * const pl080_writefn[] = {
    pl080_write,
    pl080_write,
    pl080_write