ENGR00121057 switch low power mode only support in mc13892 2.0a

BugLink: http://bugs.launchpad.net/bugs/615722

switch low power mode will cause problems on previous version of
mc13892, which may break mc13892 chip.
This is a fix for ENGR00120510.

Signed-off-by: Shen Yong <b00984@freescale.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>