drm/nouveau/volt: implement voltage control in core

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

diff --cc drivers/gpu/drm/nouveau/Makefile
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/engine/device/nv40.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/engine/device/nv50.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/engine/device/nvc0.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/engine/device/nve0.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/include/subdev/volt.h
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/subdev/volt/base.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/subdev/volt/gpio.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/subdev/volt/nv40.c
Simple merge