usb: xhci: Remove assumption of DWC instance based on DRA7 SoC type

Both AM57xx and DRA7xx share the same set of base addresses for DWC
controllers. The usage however differ with DWC2 instance used typically
in AM57xx evms while DWC1 instances used in DRA7x platforms.

Use TARGET_SOC config to differentiate so that CONFIG_AM57XX can be dropped.

Eventually, this needs to be dt-fied.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>