[sram/dv] Minor DV doc update
[sram/dv] Minor DV doc update

1. added prim_prince, prim_lfsr are verified separately
2. added using different INSTR_EXEC parameter for ret and main sram

Signed-off-by: Weicai Yang <4fa8c7762ee4da95e3dee7dae2c3263ea16637f9@google.com>

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