soc: ixp4xx: move cpu detection to linux/soc/ixp4xx/cpu.h

Generic drivers are unable to use the feature macros from mach/cpu.h
or the feature bits from mach/hardware.h, so move these into a global
header file along with some dummy helpers that list these features as
disabled elsewhere.

Cc: David S. Miller <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: netdev@vger.kernel.org
Cc: Zoltan HERPAI <wigyori@uid0.hu>
Cc: Raylynn Knight <rayknight@me.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>