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>