arm: usb: dra7xx: xHCI registers based on USB port index

arm: usb: dra7xx: xHCI registers based on USB port index

Modify the determination of the base address of xHCI registers of DRA7XX
targets.
Before the commit: by the target.
After the commit: by the USB port index.

Cc: Lokesh Vutla <lokeshvutla@ti.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Roger Quadros <rogerq@ti.com>
Signed-off-by: Uri Mashiach <uri.mashiach@compulab.co.il>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Roger Quadros <rogerq@ti.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Marek Vasut <marex@denx.de>