ARM: uniphier: enable CONFIG_MMC_SDHCI_SDMA for ARMv8 SoCs

ARM: uniphier: enable CONFIG_MMC_SDHCI_SDMA for ARMv8 SoCs

I did not enable SDMA when I added sdhci-cadence support because LD20
boards are equipped with a large amount memory beyond 32 bit address
range, but SDMA does not support the 64bit address.  U-Boot relocates
itself to the end of effectively available RAM.  This would make the
MMC enumeration fail because the buffer for EXT_CSD allocated in the
stack would go too high, then SDMA would fail to transfer data.

Recent SDHCI-compatible controllers support ADMA, but unfortunately
U-Boot does not support ADMA.

In the previous commit, I hided the DRAM area that exceeds the 32 bit
address range.  Now, I can enable CONFIG_MMC_SDHCI_SDMA.

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