Documentation: gpu: nova: document the IFR header layout

Init-from-ROM (IFR) is a special GPU feature used for power management
on some Nvidia GPUs.  It references data in the VBIOS for its operation,
but for drivers the important piece is the header that precedes the
VBIOS PCI Expansion ROM.  Parsing VBIOS is necessary to boot GSP-RM on
Turing, Ampere, and Ada GPUs.

Most such GPUs do not need to parse the IFR header in order to find the
VBIOS, but the Nvidia GA100 is the exception.  GA100 lacks a display
engine, so the PRAMIN method (which reads the VBIOS from VRAM via
display hardware) is unavailable, forcing the driver to read the ROM
directly via PROM.  On other similar GPUs, either PRAMIN succeeds before
PROM is tried, or the IFR hardware has already applied the ROM offset
so that PROM reads transparently skip the IFR header.

Note that GH100 also does not have a display engine, but it uses a
completely different method to boot GSP-RM.

This information is derived from NVIDIA's open-source GPU kernel module
driver (aka OpenRM), specifically the NV_PBUS_IFR_FMT_FIXEDx definitions
in dev_bus.h and the parsing logic in s_romImgFindPciHeader_TU102().

Signed-off-by: Timur Tabi <ttabi@nvidia.com>
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Link: https://patch.msgid.link/20260408142132.3911466-1-ttabi@nvidia.com
[acourbot: fix TOTAL_DATA_SIZE definition.]
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

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