sandbox: tests: Add tests for mc34708 PMIC device

Following tests has been added for mc34708 device:

- get_test for mc34708 PMIC
- Check if proper number of registers is read
- Check if default (emulated via i2c device) value is properly read
- Check if value write/read operation is correct
- Perform tests to check if pmic_clrsetbits() is working correctly

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>