ARM: at91/board-dt: drop default console

This default console mechanism is not used if the console is selected
by command line (console= parameter).
Dropping this will simplify the compilation of multiple SoC in the same
kernel image.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>