ARM: at91: remove CONFIG_MACH_SAMA5_DT

CONFIG_MACH_SAMA5_DT is useless as the only way to boot on sama5 based boards is
to use device tree.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
[nicolas.ferre@atmel.com: adapt on top of cleanup branch]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>