ACPI: tables: Simplify PPTT leaf node detection

ACPI 6.3 bumps the PPTT table revision and adds a LEAF_NODE flag.

This allows us to avoid a second pass through the table to assure
that the node in question is a leaf.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>