intel_idle: Add CPU model 54 (Atom N2000 series)

Add CPU ID for Atom N2600/N2800 processors. Datasheets indicate support
for this, detailed information about potential quirks or limitations are
missing, though. So we just reuse the definition for the previous ATOM
series. Tests on N2800 systems showed that this addition is fine an can
reduce power consumption by about 0.25 W (personally confirmed on Intel
DN2800MT).

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Len Brown <len.brown@intel.com>