turbostat: fix PC6 displaying on some systems

'MSR_PKG_CST_CONFIG_CONTROL' encodes the deepest allowed package C-state limit,
and turbostat decodes it.

Before this patch: turbostat does not recognize value "3" on Ice Lake Xeon
(ICX) and Sapphire Rapids Xeon (SPR), treats it as "unknown", and does not
display any package C-states in the results table.

After this patch: turbostat recognizes value 3 on ICX and SPR, treats it as
"PC6", and correctly displays package C-states in the results table.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>