doc: add missing Ethernet Support for Nucleo H563ZI & STM32H573I-DK

I recently found out, that the Ethernet support for the boards Nucleo
H563ZI and STM32H573I-DK is already available but it isn't mentioned
in the "Supported Features" section.

Signed-off-by: Philipp Steiner <philipp.steiner1987@gmail.com>