Support BIOS control of max freq setting (for HANA)

hb_cap_hana_freq_mhz_max (read-only) - provided by Hostboot during IPL
hb_cap_hana_freq_mhz_min (read-only) - provided by Hostboot during IPL
hb_cap_hana_freq_mhz_request (latched attribute) - used to override max freq

Signed-off-by: Matt Derksen <mderkse1@us.ibm.com>
Change-Id: Idc8967868d2922514e1876fa1ce67b3a58b2bf9b