Documentation: Add APM X-Gene SoC 15Gbps Multi-purpose PHY driver binding documentation

This patch adds the APM X-Gene SoC 15Gbps Multi-purpose PHY driver binding
documentation.

Signed-off-by: Loc Ho <lho@apm.com>
Signed-off-by: Tuan Phan <tphan@apm.com>
Signed-off-by: Suman Tripathi <stripathi@apm.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>