xilinx_intc: Fix writes into MER register

The MER register only has two valid bits. This is now used by
the linux kernel to auto-detect endianness, and causes Linux 3.15-rc1
and later to hang when run under qemu-microblaze. Mask valid bits before
writing the register to solve the problem.
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>
Reviewed-by: NEdgar E. Iglesias <edgar.iglesias@gmail.com>
[Edgar: Untabified]
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@gmail.com>

hw/intc/xilinx_intc.c

@@ -121,6 +121,9 @@ pic_write(void *opaque, hwaddr addr,
         case R_CIE:
             p->regs[R_IER] &= ~value; /* Atomic clear ie.  */
             break;
+        case R_MER:
+            p->regs[R_MER] = value & 0x3;
+            break;
         case R_ISR:
             if ((p->regs[R_MER] & 2)) {
                 break;