power: reset: mt6397: Check for null res pointer

The return value of platform_get_resource() needs to be checked.
To avoid use of error pointer in case that there is no suitable
resource.

Fixes: d28c74c1

 ("power: reset: add driver for mt6323 poweroff")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>