On Mon, May 26, 2014 at 07:55:47PM +0200, Hans de Goede wrote:

				power-on-seq = "gpio_reg_enable", "usleep 1000", "clk_32khz", "usleep 200";

...

Where power-on-seq would tell the mmc-core exactly how to bring up the sdio
device, using standard prefixes so that the mmc-core knows that something is
a clock / gpio / reset / whatever.

There was some work on such generic power on sequences in DT from 
Alexandre Courbot a while ago, search for "Runtime Interpreter Power
Sequences".  It ground to a halt IIRC mainly because it was never clear
that this should ever be encoded outside of the driver for a device but
the binding was pretty good and well reviewed, it's a good place to
start if there's a desire to do things like this.