target/arm: allow DC CVA[D]P in user mode emulation

target/arm: allow DC CVA[D]P in user mode emulation

DC CVAP and DC CVADP instructions can be executed in EL0 on Linux,
either directly when SCTLR_EL1.UCI == 1 or emulated by the kernel (see
user_cache_maint_handler() in arch/arm64/kernel/traps.c).

This patch enables execution of the two instructions in user mode
emulation.

Signed-off-by: Zhuojia Shen <chaosdefinition@hotmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>