[POWERPC] Add of_device_is_available function

IEEE 1275 defined a standard "status" property to indicate the operational
status of a device.  The property has four possible values: okay, disabled,
fail, fail-xxx.  The absence of this property means the operational status
of the device is unknown or okay.

This adds a function called of_device_is_available that checks the state
of the status property of a device.  If the property is absent or set to
either "okay" or "ok", it returns 1.  Otherwise it returns 0.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>