drm/nouveau: support for probing platform devices

Add a platform driver for Nouveau devices declared using the device tree
or platform data. This driver currently supports GK20A on Tegra
platforms and is only compiled for these platforms if Nouveau is
enabled.

Nouveau will probe the chip type itself using the BOOT0 register, so all
this driver really needs to do is to make sure the module is powered and
its clocks active before calling nouveau_drm_platform_probe().

Heavily based on work done by Thierry Reding.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>