arc: Add virt_to_phys() stub

arc: Add virt_to_phys() stub

Commit cf7c93cdd755 "usb: ehci: Implement V2P mapping"
introduced usage of virt_to_phys() in ehci-hcd.

Since there was no implementation of virt_to_phys() for ARC
compilation of the ehci-generic driver failed.

This change adds virt_to_phys() stub for ARC so now
USB driver for AXS101 board could be built again.

Signed-off-by: Alexey Brodkin <[email protected]>
Cc: Daniel Schwierzeck <[email protected]>
Cc: Hans de Goede <[email protected]>
Acked-by: Marek Vasut <[email protected]>