MAINTAINERS: Update the RISC-V CPU Maintainers

MAINTAINERS: Update the RISC-V CPU Maintainers

Update the RISC-V maintainers by removing Sagar and Bastian who haven't
been involved recently.

Also add Bin who has been helping with reviews.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Acked-by: Bin Meng <bin.meng@windriver.com>
Acked-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 6564ba829c40ad9aa7d28f43be69d8eb5cf4b56b.1617749142.git.alistair.francis@wdc.com