mmc: sdhci-pltfm: Add structure for host-specific data

We need to carry some information per host, e.g. the clock. Add a
structure for it and initialize it in the generic part. Also improve
the check for a parent.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Richard Röjfors <richard.rojfors.ext@mocean-labs.com>
Signed-off-by: Chris Ball <cjb@laptop.org>