ahci: Add a disk-controller uclass

Add a uclass ID for a disk controller. This can be used by AHCI/SATA or
other controller types. There are no operations and no interface so far,
but it is possible to probe a SATA device.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>