serial: sh-sci: Move private definitions to private header file

Move private register definitions and enums from the public
<linux/serial_sci.h> header file to the driver private "sh-sci.h" header
file.

The common Serial Control Register definitions are left in the public
header file, as they're needed to fill in plat_sci_port.scscr on legacy
systems not using DT.

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