RISC-V: XTheadMemPair: Remove register restrictions for store-pair

RISC-V: XTheadMemPair: Remove register restrictions for store-pair

The XTheadMemPair does not define any restrictions for store-pair
instructions (th.sdd or th.swd). However, the current code enforces
the restrictions that are required for load-pair instructions.
Let's fix this by removing this code.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-ID: <20230220095612.1529031-1-christoph.muellner@vrull.eu>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>