powerpc/85xx: add sysfs for pw20 state and altivec idle

Add a sys interface to enable/diable pw20 state or altivec idle, and
control the wait entry time.

Enable/Disable interface:
    0, disable. 1, enable.
    /sys/devices/system/cpu/cpuX/pw20_state
    /sys/devices/system/cpu/cpuX/altivec_idle

Set wait time interface:(Nanosecond)
    /sys/devices/system/cpu/cpuX/pw20_wait_time
    /sys/devices/system/cpu/cpuX/altivec_idle_wait_time
Example: Base on TBfreq is 41MHZ.
    1~48(ns): TB[63]
    49~97(ns): TB[62]
    98~195(ns): TB[61]
    196~390(ns): TB[60]
    391~780(ns): TB[59]
    781~1560(ns): TB[58]
    ...

Signed-off-by: Wang Dongsheng <dongsheng.wang@freescale.com>
[scottwood@freescale.com: change ifdef]
Signed-off-by: Scott Wood <scottwood@freescale.com>