Documentation: Add documentation for the APM X-Gene SoC SATA host controller DTS binding

This patch adds documentation for the APM X-Gene SoC SATA host controller DTS
binding.

Signed-off-by: Loc Ho <lho@apm.com>
Signed-off-by: Tuan Phan <tphan@apm.com>
Signed-off-by: Suman Tripathi <stripathi@apm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tejun Heo <tj@kernel.org>