buildroot

configs: add defconfig for Atmel sama5d3xek

This supports:
 - sama5d31ek
 - sama5d33ek
 - sama5d34ek
 - sama5d35ek
 - sama5d36ek

with or without the PDA4 or PDA7 screens.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>