MIPS: DTS: CI20: multiple DTS improvements

a) add DT node for SW1 as Enter button

The SW1 button can be used as a simple one-button keyboard
and is connected to PD17.

Note: SW1 has a second meaning to change the boot sequence
when pressed while powering on.

b) give eth0_power a defined voltage.

This is a 3.3V power switch (DVNET3.3V).

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>