serial: 8250_uniphier: add bindings document for UniPhier UART

This is binding information for the UniPhier on-chip UART driver
(drivers/tty/serial/8250/8250_uniphier.c).

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