- Remove repetitive declarations of two `bus_type` structs.
- Fix mmu.c to use an always-64-bit member for `parent_addr`.
- Remove call to `page_mapcount_reset()` that isn't needed.
- Correct `nvdebug_pd_page` documentation.
- Disable missing prototype and missing declaration warnings.
- Fix an improper type, and a missing type conversion.

Tested build on Linux 6.14.0 (x86_64), 5.4.0 (x86_64), and
4.9.337 (aarch64).

When the GPU is powered off, attempts to read any of its registers
(such as via nvdebug_readl()) result in a fatal interrupt. The
pm_runtime_get() call included in nvdebug sent a request to nvgpu
to turn the GPU back on. **However,** this call did not wait for
the power-on command to take effect. This resulted in a race between
nvdebug and the power management logic, meaning that the GPU may not
have powered-on by the time that nvdebug attempted to read its
registers.

Use pm_runtime_get_sync() instead, which explicitly waits for the
power-on command to complete (or fail) before returning. This
eliminates the race condition.

Thank you to Diego Alejandro Parra Guzman
<diego.guzman@tttech-auto.com>, who brought this issue to my
attention.

Instead of printing the read in `nvdebug_reg32_read()`, another
read was being performed, using the first read value as the register
offset! This is a mistaken incomplete removal of the old pre-error-
-handling logic in 71be6bb5.

This caused any APIs using this function to not work by returning
bizzare or incorrect values, or crashing the system on Jetson boards.

The dependency on these was removed in commit 8340d234.

For integrated (Jetson) GPUs:
- Directly retrieve and map GPU register region 0
- Directly check GPU power-on state before a register read/write
- Resume the GPU as needed for a register read/write

Most nvgpu APIs can now be called on TX2+ integrated GPUs without
first having to start some task on the GPU to make it non-suspended.

Tested on Jetson TX1, TX2, Xavier, and Orin.

See also the RTAS'23 and RTAS'24 papers.

Not known to cause any current bugs, but could cause the returned
address to be inaccessible.

Draws from NVIDIA's open-gpu-kernel-modules project.