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>

arch/arc/include/asm/io.h

@@ -239,4 +239,9 @@ static inline int __raw_writesl(unsigned int addr, void *data, int longlen)
 #define setbits_8(addr, set) setbits(8, addr, set)
 #define clrsetbits_8(addr, clear, set) clrsetbits(8, addr, clear, set)
 
+static inline phys_addr_t virt_to_phys(void *vaddr)
+{
+	return (phys_addr_t)((unsigned long)vaddr);
+}
+
 #endif	/* __ASM_ARC_IO_H */