target/arm: Allow EL change hooks to do IO

target/arm: Allow EL change hooks to do IO

During code generation, surround CPSR writes and exception returns which
call the EL change hooks with gen_io_start/end. The immediate need is
for the PMU to access the clock and icount during EL change to support
mode filtering.

Signed-off-by: Aaron Lindsay <alindsay@codeaurora.org>
Message-id: 1523997485-1905-9-git-send-email-alindsay@codeaurora.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>