ARM: 7029/1: Make cpu_architecture into a global variable

The CPU architecture really should not be changing at runtime, so
make it a global variable instead of a function.

The cpu_architecture() function declared in <asm/system.h> remains
the correct way to read this variable from C code.

Signed-off-by: Dave Martin <dave.martin@linaro.org>
Reviewed-by: Jon Medhurst <tixy@yxit.co.uk>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>