Make CPURead/WriteFunc structure 'const'

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/hw/pxa2xx_keypad.c b/hw/pxa2xx_keypad.c
index 80dcb58..060df58 100644
--- a/hw/pxa2xx_keypad.c
+++ b/hw/pxa2xx_keypad.c
@@ -257,13 +257,13 @@
     }
 }
 
-static CPUReadMemoryFunc *pxa2xx_keypad_readfn[] = {
+static CPUReadMemoryFunc * const pxa2xx_keypad_readfn[] = {
     pxa2xx_keypad_read,
     pxa2xx_keypad_read,
     pxa2xx_keypad_read
 };
 
-static CPUWriteMemoryFunc *pxa2xx_keypad_writefn[] = {
+static CPUWriteMemoryFunc * const pxa2xx_keypad_writefn[] = {
     pxa2xx_keypad_write,
     pxa2xx_keypad_write,
     pxa2xx_keypad_write