mlxsw: reg: Mark SFGC & some SFMR fields as reserved in CFF mode

Some existing fields and the whole register of SFGC are reserved in CFF
mode. Backport the reservation note to these fields.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/e1d5977a8cb778227e4ea2fd1515529957ce5de7.1700503643.git.petrm@nvidia.com


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