soc: samsung: exynos-chipid: Add NULL pointer check in exynos_chipid_probe()

In the Linux kernel, the following vulnerability has been resolved: soc: samsung: exynos-chipid: Add NULL pointer check in exynos_chipid_probe() soc_dev_attr->revision could be NULL, thus, a pointer check is added to prevent potential NULL pointer dereference. This is similar to the fix in commit 3027e7b15b02 ("ice: Fix some null pointer dereference issues in ice_ptp.c"). This issue is found by our static analysis tool.