ARM: dts: i.MX51: digi-connectcore-som: Remove unused regulators

VGEN1, VGEN2 and GPO1 regulators are not used on SOM.
Let's remove these entries.

Signed-off-by: Alexander Shiyan <eagle.alexander923@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>