gpu: nova-core: register sysmem flush page

Reserve a page of system memory so sysmembar can perform a read on it if
a system write occurred since the last flush. Do this early as it can be
required to e.g. reset the GPU falcons.

Chipsets capabilities differ in that respect, so this commit also
introduces the FB HAL.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250619-nova-frts-v6-14-ecf41ef99252@nvidia.com


[ * Use kernel::page::PAGE_SIZE instead of kernel::bindings::PAGE_SIZE.
  * Get rid of the Option for SysmemFlush.
  * Slightly reword SysmemFlush doc-comments.

  - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>