net/mlx5e: Refactor reg_c1 usage

Following patch in series uses reg_c1 in eswitch code. To use reg_c1
helpers in both TC and eswitch code, refactor existing helpers according to
similar use case of reg_c0 and move the functionality into eswitch.h.
Calculate reg mappings length from new defines to ensure that they are
always in sync and only need to be changed in single place.

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Dmytro Linkin <dlinkin@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>