[dv, chip] Update testplan for chip_sw_tap_strap_sampling
[dv, chip] Update testplan for chip_sw_tap_strap_sampling

Added some clarification and added testpoints for DFT SW straps
Signed-off-by: Weicai Yang <4fa8c7762ee4da95e3dee7dae2c3263ea16637f9@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan