sata: implement reset_sata for dwc_ahsata

Add reset_sata() to the sata driver interface and implement it
for dwc_ahsata. This function cleans up after sata_init(), and
therefore accepts a device number like sata_init() does.
A dummy implementation is provided for the rest of the drivers.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefano Babic <sbabic@denx.de>