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>