ARM: uniphier: remove CONFIG_SYS_SDRAM_BASE

The base address of DRAM was 0x80000000 for all the ARM SoCs of this
family in the past. It will be changed to 0x20000000 for a planned new
SoC. To support multiple SoCs by the single uniphier_v8_defconfig, the
base must be run-time determined.

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