usb: Add support for txfifo threshold

CONFIG_USB_EHCI_TXFIFO_THRESH enables setting of the txfilltuning
field in the EHCI controller on reset.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Remy Bohmer <linux@bohmer.net>
Signed-off-by: Tom Warren <twarren@nvidia.com>
diff --git a/README b/README
index 5452813..d4799e2 100644
--- a/README
+++ b/README
@@ -1125,6 +1125,9 @@
 				May be defined to allow interrupt polling
 				instead of using asynchronous interrupts
 
+		CONFIG_USB_EHCI_TXFIFO_THRESH enables setting of the
+		txfilltuning field in the EHCI controller on reset.
+
 - USB Device:
 		Define the below if you wish to use the USB console.
 		Once firmware is rebuilt from a serial console issue the