ARM: shmobile: wait for MSTP clock status to toggle, when enabling it

ARM: shmobile: wait for MSTP clock status to toggle, when enabling it

commit a028c6da34d434e35ba8322568c756ea97ff3c18 upstream.

On r-/sh-mobile SoCs MSTP clocks are used by the runtime PM to dynamically
enable and disable peripheral clocks. To make sure the clock has really
started we have to read back its status register until it confirms success.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Jiri Slaby <jslaby@suse.com>