irqchip/gic-its: Rework platform MSI deviceID detection

irqchip/gic-its: Rework platform MSI deviceID detection

Current code retrieving platform devices MSI devID in the GIC ITS MSI
parent helpers suffers from some minor issues:

- It leaks a struct device_node reference
- It is duplicated between GICv3 and GICv5 for no good reason
- It does not use the OF phandle iterator code that simplifies
  the msi-parent property parsing

Consolidate GIC v3 and v5 deviceID retrieval in a function that addresses
the full set of issues in one go by merging GIC v3 and v5 code and
converting the msi-parent parsing loop to the more modern OF phandle
iterator API, fixing the struct device_node reference leak in the process.

Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://patch.msgid.link/20251021124103.198419-6-lpieralisi@kernel.org