[otbn, rtl] Correctly override 'sel' signal for GPR secure wipe
[otbn, rtl] Correctly override 'sel' signal for GPR secure wipe

Otherwise, you end up grabbing whatever the final instruction decode
happened to pick.

Signed-off-by: Prajwala Puttappa <d42da9c1e7c0c7ddf8c0244ff16e57febc9395a0@lowrisc.org>

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