misc: pch_phub: Read prefetch value from device tree if passed

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>