ARM: dts: r7s72100: add RTC_X clock inputs to device tree

Add the RTC clocks to device tree. The frequencies must be fixed values
according to the hardware manual.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>