pinctrl: uniphier: add UART hardware flow control pin-mux settings

UniPhier SoCs have the following pins for hardware flow control of UART:
  XRTS, XCTS
and for modem control of UART:
  XDTR, XDSR, XDCD, XRI

The port number with the flow control is SoC-dependent.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>