drivers: counter: Add SAM0 basic counter support

This adds support for the basic timer counter (TC) found on SAM0
series parts.  This driver only supports running the counter
in 32 bit wide mode.  Since this mode explicitly slaves the odd
counters to the even ones, only instances of the even ones are
defined.

Tested with tests/drivers/counter/counter_basic_api on SAMD21.

Signed-off-by: Derek Hageman <hageman@inthat.cloud>