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

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 <[email protected]>
Cc: Stefano Babic <[email protected]>
Cc: Heiko Schocher <[email protected]>
Cc: Tim Harvey <[email protected]>
Cc: Eric Bénard <[email protected]>
Cc: Fabio Estevam <[email protected]>
Cc: Eric Nelson <[email protected]>
Cc: Marek Vasut <[email protected]>
Cc: Christian Gmeiner <[email protected]>
Cc: Stefan Roese <[email protected]>
Cc: Soeren Moch <[email protected]>
Cc: Otavio Salvador <[email protected]>
Acked-by: Stefano Babic <[email protected]>
Acked-by: Soeren Moch <[email protected]>
Acked-by: Otavio Salvador <[email protected]>
Acked-by: Christian Gmeiner <[email protected]>