arm64: defconfig: enable RX-8581 config option

Enable support for RX-8571/RX-8581 RTC by turning on
CONFIG_RTC_DRV_RX8581 as module.

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>