ARM: at91: introduce the core type choice to split ARMv4/5 and ARMv7 arch

As we will introduce SAMA5, we need to distinguish the core architecture. It is
useless to show ARMv4/5 entries if we are configuring a kernel for SAMA5
devices.

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