ARM: uniphier: use $loadaddr for source command

If the "source" command is not given the address, it uses
CONFIG_SYS_LOAD_ADDR, which is compile-time determined.

Using the "loadaddr" environment variable is handier because it is
relocated according to the memory base when CONFIG_POSITION_INDEPENDENT
is enabled.

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