perf/x86/intel/rapl: Cosmetic rename internal variables in response to multi-die/pkg support

Syntax update only -- no logical or functional change.

In response to the new multi-die/package changes, update variable names to
use "die" terminology, instead of "pkg".

For previous platforms which doesn't have multi-die, "die" is identical as
"pkg".

Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/0ddb97e121397d37933233da303556141814fa47.1557769318.git.len.brown@intel.com