ARM: imx: use generic function to exit coherency

The common ARM architecture code provides a generic function to exit
coherency called v7_exit_coherency_flush(). Replace the machine
specific implementation using the generic function.

Tested on a i.MX 6Dual by hotplugging the secondary CPU under load
through sysfs several 1000 times.

Tested-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>