ARM: at91: add sama5d4 support to sama5_defconfig

Add sama5d4 to sama5_defconfig to build kernel booting on both sama5d3 and
samad4.

Note that earlyprintk can only be working for one or the other.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>