ARM: dts: imx6*-hummingboard2: remove non-mainline property from RTC

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>