Add basic support for R-Car Salvator-X M3-N (R8A77965) board.

Based on original work from:
Takeshi Kihara <takeshi.kihara.df@renesas.com>
Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>