drm/nouveau/secboot/gm20b: specify MC base address as argument

Allow the MC base address to be specified as an argument for the WPR
region reading function. GP10B uses a different address layout as GM20B,
so this is necessary. Also export the function to be used by GP10B.

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