ata: ceva: Add gen 3 mode support in driver

This patch sets gen 3 mode as default mode in ahci_ceva driver.

Signed-off-by: Anurag Kumar Vulisha <anuragku@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Tejun Heo <tj@kernel.org>