serial: xuartps: add __init to earlycon write method

Early console functions are only used during the early boot stage.
This change just saves a small amount of memory footprint.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>