hwmon: (coretemp) Fix compile error if CONFIG_SMP is not defined

cpu_sibling_mask() is not defined unless CONFIG_SMP is defined, so it
must not be used directly in the code without ifdef protection.

To solve the problem and avoid ifdefs in the code, define
for_each_sibling() and use it instead.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>