MAINTAINERS: add myself as maintainer for b53

MAINTAINERS: add myself as maintainer for b53

I wrote the original OpenWrt driver that Florian used as the base for
the dsa driver, I might as well take responsibility for it.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/20251013180347.133246-1-jonas.gorski@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>