MAINTAINERS: Add myself for stmmac ethernet driver maintainance

MAINTAINERS: Add myself for stmmac ethernet driver maintainance

The stmmac driver based on Synopsys' dwmac IP is used in a very wide
variety of SoCs and is currently very actively used and contributed to.

It has been orphaned in January 2025 after the previous maintainers
became inactive, but Russell King was providing very valuable reviews
and fixes for the driver at that point.

Now we're seeing more and more activity on the driver, but are lacking
people to test and review contributions to both glue drivers as well as
core stmmac code.

I have access to some variety of stmmac-based platforms such as socfpga
CycloneV, imx8mp, some Allwinner SoCs and stm32mp1xx boards that I can
run regression tests on, and I'm offering to step-up as a maintainer for
driver, for the time being at least.

Let's hope other people will eventually join this effort.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20260722142125.1767689-1-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>