serial: sh-sci: Add pin initialization for SCIFA/SCIFB

Before, the driver relied on initialization by the boot loader, or by
implicit reset state.

Note that unlike on (H)SCIF, the RTS/CTS bits exist only if dedicated
RTS/CTS pins are available, which depends on the SoC and UART instance.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>