power_mgmt: Add device power management support

Added device power management hook infrastructure. Added
DEVICE_INIT_PM and SYS_INIT_PM macros that creates device
structures with the supplied device_ops structure containing
the hooks.

Added example support in gpio_dw driver.  Updated the sample
app and tested using LPS and Device Suspend Only policies.

Change-Id: I2fe347f8d8fd1041d8318e02738990deb8c5d68e
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>