soc: fsl: guts: check return value after calling of_iomap() in fsl_guts_get_soc_uid()

of_iomap() may return NULL, so we need check the return value.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>