ARM: mach-imx: Fix selection of ARCH_MXC

Since commit c5a0d497(ARM: imx: enable multi-platform build),
ARCH_MXC is selected by the following logic:

config ARCH_MXC
        def_bool y if ARCH_MULTI_V4_V5 || ARCH_MULTI_V6_V7

, which causes build error on vexpress_defconfig:

arch/arm/mach-imx/hotplug.c:49: undefined reference to `imx_enable_cpu'
arch/arm/mach-imx/platsmp.c:57: undefined reference to `imx_set_cpu_jump'
arch/arm/mach-imx/platsmp.c:58: undefined reference to `imx_enable_cpu'

Make ARCH_MXC a user selectable option, so that it does not get built
by default on other defconfigs that select ARCH_MULTI_V4_V5 or ARCH_MULTI_V6_V7.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>

arch/arm/configs/imx_v4_v5_defconfig

@@ -17,6 +17,7 @@ CONFIG_MODULE_UNLOAD=y
 # CONFIG_BLK_DEV_BSG is not set
 # CONFIG_IOSCHED_DEADLINE is not set
 # CONFIG_IOSCHED_CFQ is not set
+CONFIG_ARCH_MXC=y
 CONFIG_ARCH_MULTI_V4T=y
 CONFIG_ARCH_MULTI_V5=y
 # CONFIG_ARCH_MULTI_V7 is not set

arch/arm/configs/imx_v6_v7_defconfig

@@ -16,6 +16,7 @@ CONFIG_MODULE_UNLOAD=y
 CONFIG_MODVERSIONS=y
 CONFIG_MODULE_SRCVERSION_ALL=y
 # CONFIG_BLK_DEV_BSG is not set
+CONFIG_ARCH_MXC=y
 CONFIG_ARCH_MULTI_V6=y
 CONFIG_ARCH_MULTI_V7=y
 CONFIG_MACH_MX31LILLY=y

arch/arm/mach-imx/Kconfig

 config ARCH_MXC
-	def_bool y if ARCH_MULTI_V4_V5 || ARCH_MULTI_V6_V7
+	bool "Freescale i.MX family" if ARCH_MULTI_V4_V5 || ARCH_MULTI_V6_V7
 	select ARCH_REQUIRE_GPIOLIB
 	select ARM_PATCH_PHYS_VIRT
 	select AUTO_ZRELADDR if !ZBOOT_ROM