[prim_flash] Add reset to held_part
[prim_flash] Add reset to held_part

`held_part` has no reset. This commit is to add reset value to the
register.

Signed-off-by: Eunchan Kim <cb7bc7ceb22698f298c67faa994fd9ddf3cab298@opentitan.org>

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