The default prefetch value for the eg20t device is hard coded to
0x000affaa.

Add support for an alternative to be read from DT if available
Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>