arm64: dts: renesas: r8a774c0-cat874: add RTC support

This patch adds Epson RX-8571 real time clock support.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>