ARM: uniphier: set loadaddr at boot-time

The base of DRAM will be changed for the next generation SoC.
To support it along with existing SoCs in the single defconfig,
set 'loadaddr' at boot-time by adding the offset to the DRAM base.

CONFIG_SYS_LOAD_ADDR is still hard-coded for compilation, but the
value from environment variable 'loadaddr' should be used.

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