net/mlx5e: Move DIM function declarations to en/dim.h

Create a header specifically for DIM-related declarations. Move existing
DIM-specific functionality from en.h. Future DIM-related functionality will
be declared in en/dim.h in subsequent patches.

Co-developed-by: Nabil S. Alramli <dev@nalramli.com>
Signed-off-by: Nabil S. Alramli <dev@nalramli.com>
Co-developed-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/20240419080445.417574-2-tariqt@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>