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>
diff --git a/configs/sama5d2_xplained_spiflash_defconfig b/configs/sama5d2_xplained_spiflash_defconfig
index 41ab772..0838e41 100644
--- a/configs/sama5d2_xplained_spiflash_defconfig
+++ b/configs/sama5d2_xplained_spiflash_defconfig
@@ -42,7 +42,7 @@
 CONFIG_DM_I2C=y
 CONFIG_SYS_I2C_AT91=y
 CONFIG_DM_MMC=y
-CONFIG_ATMEL_SDHCI=y
+CONFIG_MMC_SDHCI_ATMEL=y
 CONFIG_MMC_SDHCI=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y