mmc: sdhci: Add size for caller in init+register

Add a param to allow users of sdhci_pltfm to allocate private space
in calls to sdhci_pltfm_init+sdhci_pltfm_register. This is implemented
in the same way as sdhci does for its users.
 None of the users have been migrated yet and are passing in zero to
retain their private allocation.

- todo: migrate clients to using allocation this way
- todo: remove priv variable once migration is complete

Also removed unused variable in sdhci_pltfm_init fn

Signed-off-by: Christian Daudt <csd@broadcom.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>