mmc: simplify SDHCI Kconfig dependencies

We have a number of front-end drivers for SDHCI_PLTFM, some of them
use 'select MMC_SDHCI_PLTFM', others use 'depends on'. This is
inconsistent and confusing, and in one case has also led to a
build error because of incomplete dependencies:

warning: (MMC_SDHCI_PXAV3 && MMC_SDHCI_PXAV2 && MMC_SDHCI_BCM_KONA) selects MMC_SDHCI_PLTFM which has unmet direct dependencies (MMC && MMC_SDHCI)
drivers/built-in.o: In function `sdhci_sirf_resume':
:(.text+0xaaacb4): undefined reference to `sdhci_resume_host'
drivers/built-in.o: In function `sdhci_sirf_suspend':
:(.text+0xaaacf8): undefined reference to `sdhci_suspend_host'
drivers/built-in.o: In function `sdhci_sirf_probe':
:(.text+0xaaaf44): undefined reference to `sdhci_add_host'
:(.text+0xaaaf50): undefined reference to `sdhci_remove_host'

This changes Kconfig to use 'depends on MMC_SDHCI_PLTFM' for all these
cases, to fix the build error and make the logic more logical.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>