ACPI/IORT: Fix PMCG node single ID mapping handling

ACPI/IORT: Fix PMCG node single ID mapping handling

An IORT PMCG node can have no ID mapping if its overflow interrupt is
wire based therefore the code that parses the PMCG node can not assume
the node will always have a single mapping present at index 0.

Fix iort_get_id_mapping_index() by checking for an overflow interrupt
and mapping count.

Fixes: a5bd423bc086 ("ACPI/IORT: Add support for PMCG")
Signed-off-by: Tuan Phan <tuanphan@os.amperecomputing.com>
Reviewed-by: Hanjun Guo <guoahanjun@huawei.com>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Link: https://lore.kernel.org/r/1589994787-28637-1-git-send-email-tuanphan@os.amperecomputing.com
Signed-off-by: Will Deacon <will@kernel.org>