imx: mx6 move TARGET_xx Kconfig option to mx6 specific Kconfig file

Move TARGET_xx Kconfig option based on mx6 to arch/arm/cpu/armv7/mx6/Kconfig.
Add enable "CONFIG_ARCH_MX6" for boards based on mx6.
Then we can choose target boards using "make ARCH=arm menuconfig"
with ARCH_MX6 defined.

If using original way, we have no chance to enable ARCH_MX6 when
"make menuconfig". Even define CONFIG_ARCH_MX6=y in xx_defconfig,
kconfig will complains "arch/../configs/platinum_titanium_defconfig:3:
warning: override: TARGET_PLATINUM_TITANIUM changes choice state"

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Eric Bénard <eric@eukrea.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Eric Nelson <eric.nelson@boundarydevices.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Soeren Moch <smoch@web.de>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Stefano Babic <sbabic@denx.de>
Acked-by: Soeren Moch <smoch@web.de>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Christian Gmeiner <christian.gmeiner@gmail.com>