Enable 8-byte wide MMIO for 16550 serial devices

Some drivers for the PPMC7400 PowerPC evaluation board accesses the
serial registers through the floating point unit (stfd/ldfd), which is
an 8-byte wide access. This patch enables that behavior.
Signed-off-by: NMike Nawrocki <michael.nawrocki@gtri.gatech.edu>
Message-Id: <20171106161039.32596-1-michael.nawrocki@gtri.gatech.edu>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/char/serial.c

@@ -1005,7 +1005,7 @@ static void serial_mm_write(void *opaque, hwaddr addr,
                             uint64_t value, unsigned size)
 {
     SerialState *s = opaque;
-    value &= ~0u >> (32 - (size * 8));
+    value &= 255;
     serial_ioport_write(s, addr >> s->it_shift, value, 1);
 }
 
@@ -1014,16 +1014,22 @@ static const MemoryRegionOps serial_mm_ops[3] = {
         .read = serial_mm_read,
         .write = serial_mm_write,
         .endianness = DEVICE_NATIVE_ENDIAN,
+        .valid.max_access_size = 8,
+        .impl.max_access_size = 8,
     },
     [DEVICE_LITTLE_ENDIAN] = {
         .read = serial_mm_read,
         .write = serial_mm_write,
         .endianness = DEVICE_LITTLE_ENDIAN,
+        .valid.max_access_size = 8,
+        .impl.max_access_size = 8,
     },
     [DEVICE_BIG_ENDIAN] = {
         .read = serial_mm_read,
         .write = serial_mm_write,
         .endianness = DEVICE_BIG_ENDIAN,
+        .valid.max_access_size = 8,
+        .impl.max_access_size = 8,
     },
 };