This patch enables the SFP cage on the MikroTik RouterBOARD 921GS-5HPacD
(mANTBox 15s).
The RB922UAGS-5HPacD had it already working, so the support code is
moved to the common DTSI file both devices share.
Tested on a RouterBOARD 921GS-5HPacD with a MikroTik S-53LC20D module.
Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>