ARM: uniphier: move lowlevel debug init code after page table switch

As the sLD3 Boot ROM has a complex page table, it is difficult to
set up the debug UART with enabling it.  It will be much easier to
initialize the UART port after switching over to the straight-mapped
page table.

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