i2c: i2c-cdns.c: Update driver to read fifo-depth from device tree

Enable driver to fetch the optional parameter (fifo-depth) from device
tree. If the parameter is not found in the device tree, it will use
the default value declared in the driver.

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>