MAINTAINERS: Update the devicetree documentation path of imx fec driver

Change the devicetree documentation path
to "Documentation/devicetree/bindings/net/fsl,fec.yaml"
since 'fsl-fec.txt' has been converted to 'fsl,fec.yaml' already.

Signed-off-by: Cai Huoqing <caihuoqing@baidu.com>
Link: https://lore.kernel.org/r/20211014110214.3254-1-caihuoqing@baidu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>