mlxsw: reg: Add new field to Management General Peripheral Information Register

Add new field 'max_modules_per_slot' to provide maximum number of
modules that can be connected per slot. This field will always be zero,
if 'slot_index' in query request is set to non-zero value, otherwise
value in this field will provide maximum modules number, which can be
equipped on device inserted at any slot.

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