[media] coda: fix build error by making reset control optional

make reset control optional for i.MX27

The patch "[media] coda: add reset control support" introduced a build failure
if CONFIG_RESET_CONTROLLER is disabled:

    drivers/media/platform/coda.c:3734:2: error: implicit declaration of
     function 'devm_reset_control_get'

Since not all SoCs containing CODA VPUs do have a system reset controller,
use devm_reset_control_get_optional to make it optional.

Reported-by: Shawn Guo <shawn.guo@linaro.org>
Reported-by: Olof's autobuilder <build@lixom.net>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>