drm/nouveau/hwmon: Add nouveau_hwmon_ops structure with .is_visible/.read_string

This patch introduces the nouveau_hwmon_ops structure, sets up
.is_visible and .read_string operations and adds all the functions
for these operations.
This is also a preparation for the next patches, where most of the
work is being done.
This code doesn't interacture with the old one.
It's just to make easier the review of all patches.

Signed-off-by: Oscar Salvador <osalvador.vilardaga@gmail.com>
Reviewed-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>