USB event poll support

This patch adds USB event poll support, which could be used in usbkbd
and other usb devices driver when the asynchronous interrupt
processing is supported.

Signed-off-by: Zhang Wei <wei.zhang@freescale.com
diff --git a/README b/README
index f9c1978..294f0c1 100644
--- a/README
+++ b/README
@@ -901,6 +901,9 @@
 			CONFIG_USB_CONFIG
 				for differential drivers: 0x00001000
 				for single ended drivers: 0x00005000
+			CFG_USB_EVENT_POLL
+				May be defined to allow interrupt polling
+				instead of using asynchronous interrupts
 
 - USB Device:
 		Define the below if you wish to use the USB console.