ARM: dts: imx28-tx28: add trickle-charge config for DS1339

As the DS1339 driver now supports enabling the trickle charge feature
via DTB, add the appropriate properties to utilize this feature.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>