sensor: mcp9808: Adding temperature resolution to MCP9808 sensor driver

During the driver init, the function will set the sensor resolution
based on the driver's dts variable "resolution"

The driver's device tree has been updated to include the value
"resolution".

The default is set to the highest resolution of 0.0625C.

Moved mcp9808_reg_write from mcp9808_trigger.c to mcp9808.c

This allows resue of the same function in both the trigger and
resolution functions.

Function name changed to xxx_16bit to distinquish it from the 8
bit write function that will added.

Signed-off-by: Steven Daglish <s.c.daglish@gmail.com>