thermal/x86_pkg_temp: Cleanup namespace

Any randomly chosen struct name is more descriptive than phy_dev_entry.

Rename the whole thing to struct pkg_device, which describes the content
reasonably well and use the same variable name throughout the code so it
gets readable. Rename the msr struct members as well.

No functional change.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>