layerscape: add armv7 subtarget and ls1021atwr board support

layerscape: add armv7 subtarget and ls1021atwr board support

The NXP TWR-LS1021A module is a development system based
on the QorIQ LS1021A processor.
- This feature-rich, high-performance processor module can
  be used standalone or as part of an assembled Tower System
  development platform.
- Incorporating dual Arm Cortex-A7 cores running up to 1 GHz,
  the TWR-LS1021A delivers an outstanding level of performance.
- The TWR-LS1021A offers HDMI, SATA3 and USB3 connectors as
  well as a complete Linux software developer's package.
- The module provides a comprehensive level of security that
  includes support for secure boot, Trust Architecture and
  tamper detection in both standby and active power modes,
  safeguarding the device from manufacture to deployment.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>