drm/nouveau/clk: implement stub clock subdev

drivers/gpu/drm/nouveau/Makefile

drivers/gpu/drm/nouveau/core/include/subdev/clock.h

drivers/gpu/drm/nouveau/core/subdev/clock/nv04.c

drivers/gpu/drm/nouveau/core/subdev/clock/nv40.c

drivers/gpu/drm/nouveau/core/subdev/clock/nv50.c

drivers/gpu/drm/nouveau/core/subdev/clock/nva3.c

drivers/gpu/drm/nouveau/core/subdev/clock/nvc0.c

drivers/gpu/drm/nouveau/core/subdev/device/nv04.c

drivers/gpu/drm/nouveau/core/subdev/device/nv10.c

drivers/gpu/drm/nouveau/core/subdev/device/nv20.c

drivers/gpu/drm/nouveau/core/subdev/device/nv30.c

drivers/gpu/drm/nouveau/core/subdev/device/nv40.c

drivers/gpu/drm/nouveau/core/subdev/device/nv50.c

drivers/gpu/drm/nouveau/core/subdev/device/nvc0.c

drivers/gpu/drm/nouveau/core/subdev/device/nve0.c