ila: allow configuration of identifier type

Allow identifier to be explicitly configured for a mapping.
This can either be one of the identifier types specified in the
ILA draft or a value of ILA_ATYPE_USE_FORMAT which means the
identifier type is inferred from the identifier type field.
If a value other than ILA_ATYPE_USE_FORMAT is set for a
mapping then it is assumed that the identifier type field is
not present in an identifier.

Signed-off-by: Tom Herbert <tom@quantonium.net>
Signed-off-by: David S. Miller <davem@davemloft.net>