ARM: imx_v6_v7_defconfig: Don't enable PROVE_LOCKING

For some reason that is unclear to me the i.MX defconfig for v6 and v7
boards enables PROVE_LOCKING.  Since the defconfigs are generally
intended to be more production style configurations this doesn't seem
particularly intentional so drop this from the defconfig.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>