ARM: dts: i.MX51: digi-connectcore: Move RTC from SOM to JSK

In fact, the RTC battery can only be connected outside the module,
so this patch moves the PMIC RTC property and its power from SOM dts
to JSK.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>