of/irq: Factor out parsing of interrupt-map parent phandle+args from of_irq_parse_raw()

Factor out the parsing of interrupt-map interrupt parent phandle and its
arg cells to a separate function, of_irq_parse_imap_parent(), so that it
can be used in other parsing scenarios (e.g. fw_devlink).

There was a refcount leak on non-matching entries when iterating thru
"interrupt-map" which is fixed.

Tested-by: Marc Zyngier <maz@kernel.org>
Tested-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20240529-dt-interrupt-map-fix-v2-1-ef86dc5bcd2a@kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>