The sdhci_readN accessors don't lend themselves for clean use with
readx_poll_timeout because they accept two arguments. Add
a sdhci-specific helper, so the sdhci drivers can cut down on the
timeout loop boilerplate.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>