mmc: sdhci: eliminate sdhci_of_host and sdhci_of_data

The patch migrates the use of sdhci_of_host and sdhci_of_data to
sdhci_pltfm_host and sdhci_pltfm_data, so that the former pair can
be eliminated.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Grant Likely <grant.likely@secretlab.ca>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>