Speed up the boot process by using the asynchronous probing feature
supported by the recent kernels.

For SBCs based on the Actions Semi S500 SoC, the overall boot time is
expected to be reduced by 200-300 ms.
Suggested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>