target/riscv: add a MAINTAINERS entry for XThead* extension support

target/riscv: add a MAINTAINERS entry for XThead* extension support

The XThead* extensions are maintained by T-Head and VRULL.
Adding a point of contact from both companies.

Signed-off-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Message-Id: <20230131202013.2541053-15-christoph.muellner@vrull.eu>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>