mmc: sdhci-brcmstb: constify sdhci_pltfm_data structures

The sdhci_pltfm_data structure is only passed as the second argument
of sdhci_pltfm_init, which is const, so the sdhci_pltfm_data structure
can be const as well.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>