bios: Converting bios json keys to lower snake case

Converting following bios attribute json keys to lower snake case
displayName --> display_name
helpText    --> help_text
readOnly    --> read_only

Tested:
     Power on/off
     Gtest passed

Change-Id: Iad88ed7bc381d66182c99e31e7d385f65750e0ed
Signed-off-by: Archana Kakani <archana.kakani@ibm.com>