gpio: pxa: change initcall level second attempt

This patch is a second attempt at what was previously in commit
6c7e660a ("gpio: pxa: set initcall level to module init").

The goal is the same : enable gpio & pinctrl driver to work
together. As pinctrl driver will be initialized at device level, the
gpio should be as well, so that the deferring mechanism is honored.

Yet this patch should also respect the legacy platforms, so the set of
constraints is :
 - in legacy platforms (ie. non dt), gpio_[gs]et_*() should be available
   for machine code => core initcall
 - in new platforms (ie. dt based), pinctrl will be available and no
   machine code => device initcall

In order to fullfill all these constraints, the initcall level is either
postcore for non devicetree platforms, and device for devicetree platforms.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>