mx6: remove SYS_SOC from board Kconfig

mx6: remove SYS_SOC from board Kconfig

Remove duplicated SYS_SOC Kconfig entry from board Kconfig,
because we have this entry in arch/arm/cpu/armv7/mx6/Kconfig.

Signed-off-by: Peng Fan <[email protected]>
Cc: Stefano Babic <[email protected]>
Cc: Heiko Schocher <[email protected]>
Cc: Christian Gmeiner <[email protected]>
Cc: Stefan Roese <[email protected]>
Cc: Troy Kisky <[email protected]>
Cc: Nikita Kiryanov <[email protected]>
Cc: "Eric Bénard" <[email protected]>
Cc: Fabio Estevam <[email protected]>
Cc: Tim Harvey <[email protected]>
Cc: Marek Vasut <[email protected]>
Cc: Markus Niebel <[email protected]>
Cc: Otavio Salvador <[email protected]>
Acked-by: Stefan Roese <[email protected]>
Acked-by: Marek Vasut <[email protected]>
Acked-by: Christian Gmeiner <[email protected]>
Acked-by: Markus Niebel <[email protected]>
Acked-by: Troy Kisky <[email protected]>
Acked-by: Igor Grinberg <[email protected]>