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.