Change enum values to start with 1

Enum values currently start with 0, which differs from the value
used in the serialization library. While the values aren't
mandatory, it is less confusing to have the TOSA provided values
align.

Rename two enumerations for consistency:
nan_propagation_t to nan_propagation_mode_t
rounding_t to rounding_mode_t

Signed-off-by: Eric Kunze <eric.kunze@arm.com>
Change-Id: If3c8e958351a77973b910e061f270c6cf8a6dae8