README: Drop CONFIG_SYS_USB_BRG_CLK

This is not used in U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: York Sun <york.sun@nxp.com>
diff --git a/README b/README
index c4585d2..f4892fa 100644
--- a/README
+++ b/README
@@ -1481,10 +1481,6 @@
 				Derive USB clock from external clock "blah"
 				- CONFIG_SYS_USB_EXTC_CLK 0x02
 
-				CONFIG_SYS_USB_BRG_CLK 0xBLAH
-				Derive USB clock from brgclk
-				- CONFIG_SYS_USB_BRG_CLK 0x04
-
 		If you have a USB-IF assigned VendorID then you may wish to
 		define your own vendor specific values either in BoardName.h
 		or directly in usbd_vendor_info.h. If you don't define