net: ipa: identify resource groups

Define a new ipa_resource_group_id enumerated type, whose members
have numeric values that match the resource group number used when
programming the hardware.  Each platform supports a different number
of source and destination resource groups, so define the type
separately for each platform in its configuration data file.

Use these new symbolic values when specifying the resource group an
endpoint is associated with.  And use them to index the limits
arrays for source and destination resources, making it clearer how
these values are used.

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