MAINTAINERS: add Artyom Tarasenko as SPARC maintainer

MAINTAINERS: add Artyom Tarasenko as SPARC maintainer

Artyom has been working on QEMU's SPARC emulation for several years, providing
initial support for Solaris under qemu-system-sparc and more recently bugfixes
for qemu-system-sparc64 and TCG patch reviews. As work progresses on improving
emulation for sun4u machines and beyond, Artyom has agreed to take on
co-maintainership of SPARC with a focus on 64-bit architecture.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Acked-by: Artyom Tarasenko <atar4qemu@gmail.com>