serial: sh-sci: add FIFO trigger bits

Defines the bits controlling FIFO thresholds, adds the additional
HSCIF registers to the register map.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>