xen:arm: earlyprintk configuration for R-Car Gen3 boards

xen:arm: earlyprintk configuration for R-Car Gen3 boards

Introduce an earlyprintk configuration for R-Car Gen3 SoC based development
boards, like:
 - Salvator-X [http://elinux.org/R-Car/Boards/Salvator-X]
 - M3ULCB [http://elinux.org/R-Car/Boards/M3SK]
 - H3ULCB [http://elinux.org/R-Car/Boards/H3SK]

Signed-off-by: Iurii Konovalenko <iurii.konovalenko@globallogic.com>
Signed-off-by: Iurii Mykhalskyi <iurii.mykhalskyi@globallogic.com>
Signed-off-by: Andrii Anisov <andrii_anisov@epam.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>