mlxsw: reg: Extend MGPIR register with new slot fields

Extend MGPIR (Management General Peripheral Information Register) with
new fields specifying the slot number and number of the slots available
on system. The purpose of these fields is:
- to support access to MPGIR register on modular system for getting the
  number of cages, equipped on the line card, inserted at specified
  slot. In case slot number is set zero, MGPIR will provide the
  information for the main board. For Top of the Rack (non-modular)
  system it will provide the same as before.
- to provide the number of slots supported by system. This data is
  relevant only in case slot number is set zero.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>