ARM: dts: at91: sama5d3_xplained: add pincontrol for USB Host

The pincontrol node is needed for USB Host since Linux v5.7-rc1. Without
it the driver probes but VBus is not powered because of wrong pincontrol
configuration.

Fixes: b7c2b615 ("ARM: at91: add Atmel's SAMA5D3 Xplained board")
Signed-off-by: Cristian Birsan <cristian.birsan@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Link: https://lore.kernel.org/r/20201118120019.1257580-4-cristian.birsan@microchip.com