dt-bindings: i2c: i2c-cdns.txt: Add description for an optional parameter, fifo-depth

Add description for fifo-depth parameter that can be used
in the device tree.

Signed-off-by: Pei Yue Ho <peiyue.ho@starfivetech.com>
Reviewed-by: Wei Liang Lim <weiliang.lim@starfivetech.com>
Reviewed-by: Eng Lee Teh <englee.teh@starfivetech.com>
Reviewed-by: Heiko Schocher <hs@denx.de>