[doc] Update owner keys in ownership transfer
[doc] Update owner keys in ownership transfer

Update the details of the Owner Keys to align the specification with the
latest status of secure boot.

CODE_SIGN keys match the type supported by ROM/ROM_EXT sigverify module.
NEXT_OWNER and UNLOCK keys support ECC keys. Ownership transfer flows
are not expected to have strict performance requirements, thus ECC keys
are a better option. Locking down the key types allow us to finalize the
specification details of the ROM_EXT.

Signed-off-by: Miguel Osorio <c7c97b215f812c2e8a32efd57ad5e976a6eb9e45@google.com>

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