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>

arch/arm/mach-ixp4xx/Kconfig

@@ -3,28 +3,17 @@ menuconfig ARCH_IXP4XX
 	bool "IXP4xx-based platforms"
 	depends on ARCH_MULTI_V5
 	depends on CPU_BIG_ENDIAN
+	select ARM_APPENDED_DTB # Old Redboot bootloaders deployed
 	select CPU_XSCALE
 	select GPIO_IXP4XX
 	select GPIOLIB
 	select FORCE_PCI
+	select I2C
+	select I2C_IOP3XX
 	select IXP4XX_IRQ
 	select IXP4XX_TIMER
 	select USB_EHCI_BIG_ENDIAN_DESC
 	select USB_EHCI_BIG_ENDIAN_MMIO
-	help
-	  Support for Intel's IXP4XX (XScale) family of processors.
-
-if ARCH_IXP4XX
-
-config MACH_IXP4XX_OF
-	bool
-	prompt "Device Tree IXP4xx boards"
-	default y
-	select ARM_APPENDED_DTB # Old Redboot bootloaders deployed
-	select I2C
-	select I2C_IOP3XX
 	select USE_OF
 	help
-	  Say 'Y' here to support Device Tree-based IXP4xx platforms.
-
-endif
+	  Support for Intel's IXP4XX (XScale) family of processors.