ARM: dts: rskrza1: set rtc_x1 clock value

Enable the 32.768 kHz RTC_X1 clock by setting the frequency value to
non-zero.

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>