For consistency and public understanding, rework to change
'LowPowerS0Idle' to 'S0ix'.
- rename LowPowerS0Idle to S0ix
- enable s0ix by default for TGL-U
- add s0ix variable in PlatformData.h
- add s0ix flag check in stage 1B
- move Tcc s0ix support flag from stage 2 to stage 1B
Signed-off-by: Lean Sheng Tan <lean.sheng.tan@intel.com>