drm/nouveau/secboot/gm20b: add secure boot support

Add secure boot support for the GM20B chip found in Tegra X1. Secure
boot on Tegra works slightly differently from desktop, notably in the
way the WPR region is set up.

In addition, the firmware bootloaders use a slightly different header
format.

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