PHY: add APM X-Gene SoC 15Gbps Multi-purpose PHY driver

This patch adds support for the APM X-Gene SoC 15Gbps Multi-purpose PHY.
This is the physical layer interface for the corresponding host
controller. Currently, only external clock and Gen3 SATA mode
are supported.

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>