stmhal: Allow to set bits resolution for DAC; 8 is default, can have 12.
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.