x86/platform/intel-mid: Use common power off sequence

Intel Medfield may use common for Intel MID devices power sequence.
Remove unneded custom power off stub.

While here, remove function forward declaration.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170308112422.67533-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>