ARM: dts: r8a7742-iwg21m: Add RTC support

Enable BQ32000 RTC which is connected to I2C0 on the iWave RainboW-G21M-q7
SOM.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Link: https://lore.kernel.org/r/20200816193316.7641-3-prabhakar.mahadev-lad.rj@bp.renesas.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>