sata: dwc_ahsata: implement sata_port_status

Define the new common function sata_port_status() which can be
used to query the sata driver for the state of ports, and implement it
for dwc_ahsata.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Marek Vasut <marex@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>