[kmac] Revise CFG to be read-only in HW
[kmac] Revise CFG to be read-only in HW

This commit is to fix the CFG register behavior. It was external and
`hrw` type for HW to direct control the write permission.  After
utilizing REGWEN feature, it doesn't have to be write-able from HW view.
The permission was remained and caused the unknown value to kmac_en and
rest of the values, which creates an error on FPV.

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

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