ahci: Optimise AHCI controller reset and start-up

The existing code waits a whole second for the AHCI controller to reset.
Instead, let's poll the status register to see if the reset has
succeeded and return earlier if possible. This brings down the time for
AHCI probing from 1s to 20ms.

Signed-off-by: Stefan Reinauer <reinauer@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>