[otbn] improve P-384 Barrett multiplication
[otbn] improve P-384 Barrett multiplication

This improves the P-384 Barrett multiplication/reduction
subroutine w.r.t. to register usage and clobbering.

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

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