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 <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Troy Kisky <troy.kisky@boundarydevices.com>
Cc: Nikita Kiryanov <nikita@compulab.co.il>
Cc: "Eric Bénard" <eric@eukrea.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Markus Niebel <Markus.Niebel@tq-group.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Acked-by: Markus Niebel <Markus.Niebel@tq-group.com>
Acked-by: Troy Kisky <troy.kisky@boundarydevices.com>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>