target/arm: Convert load/store-pair to decodetree

Convert the load/store register pair insns (LDP, STP,
LDNP, STNP, LDPSW, STGP) to decodetree.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20230602155223.2040685-12-peter.maydell@linaro.org
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>