x86/platform/intel-mid: Remove unused leftovers (msic_power_btn)

There is no driver present, remove the device creation and other
leftovers.

Note, for Intel Merrifield there is another driver which is
instantiated by a certain MFD one and does not need any support from
device_libs.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>