[otbn] document code of P-256 ECC lib
[otbn] document code of P-256 ECC lib

Signed-off-by: Felix Miller <43d2a12732a784b305469416c0ba5ce1795e752a@gi-de.com>

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