Adjust mbedtls_mpi_mul_mpi() to new signature of mpi_mul_hlp()
Adjust mbedtls_mpi_mul_mpi() to new signature of mpi_mul_hlp()

The previous commit has changed the signature of mpi_mul_hlp(),
making the length of the output explicit.

This commit adjusts the call-site in mbedtls_mpi_mul_mpi() to
this new signature.

A notable change to the multiplication strategy had to be made:
mbedtls_mpi_mul_mpi() performs a simple row-wise schoolbook
multiplication, which however was so far computed iterating
rows from top to bottom. This leads to the undesirable consequence
that as lower rows are calculated and added to the temporary
result, carry chains can grow. It is simpler and faster to
iterate from bottom to top instead, as it is guaranteed that
there will be no carry when adding the next row to the previous
temporary result: The length of the output in each iteration
can be fixed to len(B)+1.

Signed-off-by: Hanno Becker <4d488caff78d6184f59bbe6e137e34d4606b9fe8@arm.com>

Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls