gpu: nova-core: firmware: fix and explain v2 header offsets computations

gpu: nova-core: firmware: fix and explain v2 header offsets computations

There are no offsets in `FalconUCodeDescV2` to give the non-secure and
secure IMEM sections start offsets relative to the beginning of the
firmware object.

The start offsets for both sections were set to `0`, but that is
obviously incorrect since two different sections cannot start at the
same offset. Since these offsets were not used by the bootloader, this
doesn't prevent proper function but is incorrect nonetheless.

Fix this by computing the start of the secure IMEM section relatively to
the start of the firmware object and setting it properly. Also add and
improve comments to explain how the values are obtained.

Fixes: dbfb5aa41f16 ("gpu: nova-core: add FalconUCodeDescV2 support")
Reviewed-by: Eliot Courtney <ecourtney@nvidia.com>
Link: https://patch.msgid.link/20260306-turing_prep-v11-9-8f0042c5d026@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>