ACPI/IORT: Add single mapping function

The current IORT id mapping API requires components to provide
an input requester ID (a Bus-Device-Function (BDF) identifier for
PCI devices) to translate an input identifier to an output
identifier through an IORT range mapping.

Named components do not have an identifiable source ID therefore
their respective input/output mapping can only be defined in
IORT tables through single mappings, that provide a translation
that does not require any input identifier.

Current IORT interface for requester id mappings (iort_node_map_rid())
is not suitable for components that do not provide a requester id,
so it cannot be used for IORT named components.

Add an interface to the IORT API to enable retrieval of id
by allowing an indexed walk of the single mappings array for
a given component, therefore completing the IORT mapping API.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Tomasz Nowicki <tn@semihalf.com>
Tested-by: Hanjun Guo <hanjun.guo@linaro.org>
Tested-by: Tomasz Nowicki <tn@semihalf.com>
Cc: Hanjun Guo <hanjun.guo@linaro.org>
Cc: Tomasz Nowicki <tn@semihalf.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Signed-off-by: Will Deacon <will.deacon@arm.com>