doc: dt-binding: ci-hdrc-usb2: improve property description

Improve the description of properties "tx-burst-size-dword"
and "rx-burst-size-dword".

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Shanw Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/usb/ci-hdrc-usb2.txt

@@ -41,11 +41,15 @@ Optional properties:
 - tx-burst-size-dword: it is vendor dependent, the tx burst size in dword
   (4 bytes), This register represents the maximum length of a the burst
   in 32-bit words while moving data from system memory to the USB
-  bus, changing this value takes effect only the SBUSCFG.AHBBRST is 0.
+  bus, the value of this property will only take effect if property
+  "ahb-burst-config" is set to 0, if this property is missing the reset
+  default of the hardware implementation will be used.
 - rx-burst-size-dword: it is vendor dependent, the rx burst size in dword
   (4 bytes), This register represents the maximum length of a the burst
   in 32-bit words while moving data from the USB bus to system memory,
-  changing this value takes effect only the SBUSCFG.AHBBRST is 0.
+  the value of this property will only take effect if property
+  "ahb-burst-config" is set to 0, if this property is missing the reset
+  default of the hardware implementation will be used.
 - extcon: phandles to external connector devices. First phandle should point to
   external connector, which provide "USB" cable events, the second should point
   to external connector device, which provide "USB-HOST" cable events. If one