mmc: sdhci_pltfm: Constify sdhci_pltfm_data

The sdhci_pltfm_data struct is never modified within the sdhci_pltfm
module. So make the pdata parameter to sdhci_pltfm_init and
sdhci_pltfm_register const.  This allows drivers to declare their
sdhci_pltfm_data struct as const.

This patch also makes the sdhci_pltfm_data declarations const where
possible.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>