ARM: configs: at91: add MCAN driver to sama5_defconfig

Add the Controller Area Network (MCAN) driver compilation
for sama5 default configuration. Is used by sama5d2 SoC for instance.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>