ARM: dts: imx28: Use correct compatible string for RTC

The "mxim" vendor prefix does not exit, though it looks like it might
have been a typo'ed "maxim". Still, no compatible string exists that
matches "maxim,ds1339". The closest is the "dallas,ds1339" and since
this node is named "rtc" that's probably the one that was intended.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>