serial: sam0: add samd20 support

There are some minor differences between the UART om SAMD20 and SAMD21
that we need to take into account:

SYNCBUSY bit for the samd20 is located in the STATUS reg.

The samd20 does not have a SAMPR bit like the samd21.

Signed-off-by: Sean Nyekjaer <sean@nyekjaer.dk>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>