[dv/chip] Update chip level testplan
[dv/chip] Update chip level testplan

Add more descriptions to clarify `lc_ctrl_transition` test and
`lc_walkthough` test.

Signed-off-by: Cindy Chen <399ad771a111eac8923348b1e2af093034e5d4f1@opentitan.org>

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