This patch allows to configure the DAC resolution in the constructor and
in the init function, eg:

dac = DAC(1, bits=12).

The default resolution is 8 bits for backwards compatibility.  The bits
sets the maximum value accepted by write and write_timed methods, being
2**bits - 1.

When using write_timed with 12-bit resolution, the input buffer is
treated as an unsigned half-word array, typecode 'H'.

See PR #1130 for discussion.