The SDHI block on the ag5evm requires waiting for idle
before writing to some registers.

Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>