MAINTAINERS: Update the maintainer for the CHRP NVRAM section

I am not involved in the ppc machines anymore (the pseries machine was the reason why I took over maintainership of the CHRP NVRAM code in the past), so it does not make much sense that I'm still listed here as the maintainer. The CHRP NVRAM code is used by the mac99 / g3beige ppc machines and some Sparc machines, too, where Mark is the maintainer, so I asked him whether he would be interested in being listed as the maintainer here, and fortunately, he agreed! Thanks, Mark! Signed-off-by: Thomas Huth <thuth@redhat.com> Acked-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk> Message-ID: <20260305192223.6214-1-thuth@redhat.com> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>