drivers: ethernet: stm32: Enabling HW checksum offloading for STM32H7.

This change adds support for enabling ethernet MAC hardware checksum
offloading for STM32H7 based devices.

In Section 58.5.9 of the STM32H7 reference manual it mentions that
the STM32H7 ethernet MAC supports a Checksum Offload Module (COE).

I have tested the changes on my end where I enabled
CONFIG_ETH_STM32_HW_CHECKSUM and ensured that an application that
runs Zephyr on the STM32H7 can interoperate with a device with a
completely different implementation. Also, I deliberately made
the software not populate the IPv4 and UDP header checksum fields
in their respective headers and the COE was able to populate the
IPv4 and UDP header checksums.

Given that CONFIG_ETH_STM32_HW_CHECKSUM is not enabled by default
application developers have the option to either enable it or
disable it.

Signed-off-by: Chamira Perera <chamira.perera@audinate.com>