mmc: sdhci-st: use sdhci_pltfm_init for private allocation

Commit 0e748234 ("mmc: sdhci: Add size for caller in init+register")
allows users of sdhci_pltfm to allocate private space in calls to
sdhci_pltfm_init+sdhci_pltfm_register. This patch migrates sdhci-st
to this allocation.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>