Can you please pass on whatever logs you have in /tmp after a failed attempt to install the driver with add_kernel_support script ? we might be able to pinpoint what's blocking the installation.
Looking over the release notes, i don't see a support for SLES12 SP3. the latest release of Mellanox (2.4.XX) supports SLES12 default.
Could it be that you are looking for SLES11 SP3? (the kernel version you provided also suggests its a SLES11)
Also, I see that you have a real-time kernel installed. given this is where you want to go, i would recommend you to stay with the OFED that was provided with the kernel itself (e.g. inbox driver).