dm: ahci: Unwind the confusing init code

Two AHCI drivers use SCSI with CONFIG_DM_SCSI. The SCSI uclass calls
scsi_low_level_init() which is implemented by ahci.c. If
CONFIG_SCSI_AHCI_PLAT is defined it does one thing and if it is not
it does something else.

We don't need to call through scsi_low_level_init() to get the init
completed. Instead, adjust the two drivers to call into AHCI directly.
Drop the post-probe init in the SCSI uclass. This means that driver model
doesn't need to use scsi_low_level_init(). It is a legacy function and
driver model should use a driver's probe() method instead.

While we are here, add a comment to the top of the file explaining what
ahci.c does.

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

include/ahci.h

@@ -179,4 +179,18 @@ struct ahci_uc_priv {
 int ahci_init(void __iomem *base);
 int ahci_reset(void __iomem *base);
 
+/**
+ * achi_init_one_dm() - set up a single AHCI port
+ *
+ * @dev: Controller to init
+ */
+int achi_init_one_dm(struct udevice *dev);
+
+/**
+ * achi_start_ports_dm() - start all AHCI ports for a controller
+ *
+ * @dev: Controller containing ports to start
+ */
+int achi_start_ports_dm(struct udevice *dev);
+
 #endif