arc: Add virt_to_phys() stub

Commit cf7c93cd "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 <abrodkin@synopsys.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Acked-by: Marek Vasut <marex@denx.de>