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
Cc: Stefano Babic
Cc: Heiko Schocher
Cc: Christian Gmeiner
Cc: Stefan Roese
Cc: Troy Kisky
Cc: Nikita Kiryanov
Cc: "Eric Bénard"
Cc: Fabio Estevam
Cc: Tim Harvey
Cc: Marek Vasut
Cc: Markus Niebel
Cc: Otavio Salvador
Acked-by: Stefan Roese
Acked-by: Marek Vasut
Acked-by: Christian Gmeiner
Acked-by: Markus Niebel
Acked-by: Troy Kisky
Acked-by: Igor Grinberg