xtensa: register gpio chip before use

Platform initialization sets up the LED heartbeat that is controlled
via GPIO.  Requesting the GPIO pins fails, however, as the chip is
only initialized later by a device_initcall().

Fix this up by exporting the initialization function.  Let the
platform set up the chip before it starts using it.

Signed-off-by: Johannes Weiner <jw@emlix.com>
Signed-off-by: Chris Zankel <chris@zankel.net>