dts: arm: sam0: move contents samd21 to samd

samd21.dtsi includes now the same content need for SAMD20,
move it to samd.dtsi and include it from samd21.dtsi.
Then later USB support can be added to the samd21.dtsi seperatly
from the samd20 etc.

Signed-off-by: Sean Nyekjaer <sean@nyekjaer.dk>