[ARM] 3947/1: AT91: cpu_is_XXX macro's

This patch replaces the arch_identify() in system.h with a set of
cpu_is_XXX() macro's.  This allows for compile-time checking of the
target AT91 processor.

Original patch from David Brownell.

Signed-off-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>