mmc: rockchip: rename CONFIG_ROCKCHIP_SDHCI to CONFIG_MMC_SDHCI_ROCKCHIP

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

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

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