ARM: configs: at91: sama5: PIOBU as built-in

Include as built-in as it's used by multiple boards.

Signed-off-by: Razvan Stefanescu <razvan.stefanescu@microchip.com>
[nicolas.ferre@microchip.com: adap to newer kernel]
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>