ARM: dts: am335x-wega: Clean up regulators

Cleaned up the regulators on the wega board. Created a simple bus,
renamed the regulators according to the schematics and added missing
regulator on wega.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>