realtek: switch RTL838X/RTL839X DT to new clock driver

realtek: switch RTL838X/RTL839X DT to new clock driver

Use new DT clockdriver syntax for RTL838X/RTL839X targets. To make it work
we need to change some nodes:
- define the external oscillator speed (25MHz)
- define SRAM
- add clock controller
- Add second CPU for RTL839X
- map all devices to new clocks
- Remove dummy LXB clock
- add CPU OPP table

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>