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>
diff --git a/drivers/block/ata_piix.c b/drivers/block/ata_piix.c
index 5cf91ad..3042684 100644
--- a/drivers/block/ata_piix.c
+++ b/drivers/block/ata_piix.c
@@ -192,6 +192,11 @@
 	return 0;
 }
 
+int reset_sata(int dev)
+{
+	return 0;
+}
+
 static inline u8 sata_inb(unsigned long ioaddr)
 {
 	return inb(ioaddr);