ARM: uniphier: update README.uniphier for latest build instruction

Since commit c0efc314 ("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>