AM35x: Add missing GPIO mux config for EHCI port

Adding GPIO mux config used for PHY reset of EHCI port on base board.
We get below failure message without this patch,

"hub 1-0:1.0: unable to enumerate USB device on port 1"
Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/mach-omap2/board-am3517evm.c

@@ -301,6 +301,9 @@ static void __init am3517_evm_init(void)
 				ARRAY_SIZE(am3517_evm_devices));
 
 	omap_serial_init();
+
+	/* Configure GPIO for EHCI port */
+	omap_mux_init_gpio(57, OMAP_PIN_OUTPUT);
 	usb_ehci_init(&ehci_pdata);
 	/* DSS */
 	am3517_evm_display_init();