mmc: sdhci-of-esdhc: factor out common stuff

Put everything which can be shared between the OF and platform version
of this driver into a local .h file.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Tested-by: Eric Bénard <eric@eukrea.com>
[cjb: fix compile error: sdhci-esdhc.c->sdhci-esdhc.h]
Signed-off-by: Chris Ball <cjb@laptop.org>