bcm2711/mailbox: Invalidate cache before read

Cache must be invalidated before the buffer is read since the
VideoCore's write will not invalidate the cache.

Signed-off-by: Matteo Golin <matteo.golin@gmail.com>

arch/arm64/src/bcm2711/bcm2711_mailbox.c

@@ -282,6 +282,8 @@ static int bcm2711_mbox_sendreq(FAR uint32_t *buf, uint8_t n)
    * docs).
    */
 
+  up_invalidate_dcache(bufptr, bufptr + n); /* So we read fresh data */
+
   if (retbuf != bufptr)
     {
       ipcerr("Expected %08x, got %08x", bufptr, res);