ARM: dts: imx6q-dhcom: fix rtc compatible

The only correct and documented compatible string for the rv3029 is
microcrystal,rv3029. Fix it up.

Fixes: 52c7a088 ("ARM: dts: imx6q: Add support for the DHCOM iMX6 SoM and PDK2")
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>