mmc: atmel: rename CONFIG_ATMEL_SDHCI to CONFIG_MMC_SDHCI_ATMEL

Make the naming scheme consistent; all SDHCI-base drivers prefixed
with CONFIG_MMC_SDHCI_.

While we are here, add "depends on ARCH_AT91".

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>