ARM: uniphier: update README.uniphier for latest build instruction
Since commit c0efc3140e ("ARM: uniphier: change CONFIG_SPL_PAD_TO
to 128KB"), the u-boot.bin should be burned at the offset 0x20000.
I missed to update README.uniphier in that commit. Now updating.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>