net: ipa: combine resource type definitions

Combine the ipa_resource_type_src and ipa_resource_type_dst
enumerated types into a single enumerated type, ipa_resource_type.

Assign value 0 to the first element for the source and destination
types, so their numeric values are preserved.  Add some additional
commentary where these are defined, stating explicitly that code
assumes the first source and first destination member must have
numeric value 0.

Signed-off-by: Alex Elder <elder@linaro.org>