venus: hfi_venus: add suspend functionality for Venus 4xx

This adds suspend (power collapse) functionality by reusing
the suspend function for Venus 3xx and also enables idle indicator
property for Venus 4xx (where it is disabled by default).

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>