Add support for Chromebook2 -- next-generation 8-core
(4 in operation), 4GB ram (3.5 usable) ARM machine.

Support covers device drivers for:
- Serial Peripheral Interface (SPI)
- Chrome Embedded Controller (EC) - SPI-based version
- XHCI and USB 3.0 dual-role device PHY

Also:
- Add support for Exynos5420 in Pad module
- Move power-related functions to separate driver --
  Power Management Unit (PMU)
- Enable XHCI for Chromebook1

Special thanks to grehan@ for hardware, and to
hselasky@ for r269139.