ARM: dts: iwg20d-q7: Add RTC support

Define the iWave RainboW-G20D-Qseven board dependent part of the
RTC device node.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Chris Paterson <chris.paterson2@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>