MAINTAINERS: Add Maximilian Brune to RISC-V

MAINTAINERS: Add Maximilian Brune to RISC-V

I also add myself as the Maintainer for the SiFive boards, since I
happen to have both of them and I also ported the HiFive Unmatched to
coreboot.

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: Ic0b8e1053c9f5007e29e997c1ff21ff4a496aea8
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83697
Reviewed-by: ron minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>