x86: Introduce minimal PMU driver for Intel MID platforms

This simple PMU driver allows to tyrn power on and off for selected
devices. In particularly Intel Tangier needs to power on SDHCI
controllers in order to access to them during board initialization.

In the future it might be expanded to cover other Intel MID platforms,
that's why it's located under arch/x86/lib and called pmu.c.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>