serial: sh-sci: Add BRG register definitions

Add register definitions for the Baud Rate Generator for External Clock
(BRG), as found in some SCIF and in HSCIF, including a new regtype for
the "SH-4(A)"-derived SCIF variant with BRG.

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