arm: mx6: kconfig: don't select CPU_V7 per board

CPU_V7 is already selected by ARCH_MX6, so no point in selecting it again
by boards that depend on ARCH_MX6.

Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>