ARM: ixp4xx: Consolidate Kconfig fixing issue

The IXP4xx Kconfig we ended up with for mach-ixp4xx creates
as kismet warning:

   WARNING: unmet direct dependencies detected for GPIO_IXP4XX
     Depends on [n]: GPIOLIB [=y] && HAS_IOMEM [=y] && ARCH_IXP4XX [=y] && OF [=n]
     Selected by [y]:
     - ARCH_IXP4XX [=y] && <choice>

This is because it is possible to select ARCH_IXP4XX witout
OF while that selects the GPIO driver that now depends on
OF.

Fix this by creating a single ARCH_IXP4XX kconfig that selects
USE_OF.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Krzysztof Halasa <khalasa@piap.pl>
Link: https://lore.kernel.org/r/20220522072356.34062-1-linus.walleij@linaro.org

'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>