at91: Kconfig simplification

Instead of adding "depends on" at config level, introduce HAVE_* config
variables. Add them at machine or soc level to specify the ability of a
particular support.
It will ease new board introduction and readability.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Andrew Victor <linux@maxim.org.za>