RISC-V: Add initial support for T-Head C906

RISC-V: Add initial support for T-Head C906

This patch adds the T-Head C906 to the list of known CPUs.
Selecting this CPUs will automatically enable the available
ISA extensions of the CPUs (incl. vendor extensions).

Co-developed-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: <20230131202013.2541053-13-christoph.muellner@vrull.eu>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>