serial: sh-sci: Add earlycon for R7S9210

Since the register offsets are different for RZ/A2 SCIF, we need
to declare a separate string for it.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>