irqchip/gicv3-its: platform-msi: Refactor its_pmsi_prepare()

By adding ACPI support for platform MSI, the gicv3 driver has to
provide code to retrieve the dev id through ACPI instead of device
tree bindings; given that its_pmsi_prepare() allows already to get
the dev id but it is OF dependent, factor OF related code out into
a single function to make its_pmsi_prepare() ready to be used with
other firmware interfaces.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
[lorenzo.pieralisi@arm.com: rewrote commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Tested-by: Ming Lei <ming.lei@canonical.com>
Tested-by: Wei Xu <xuwei5@hisilicon.com>
Tested-by: Sinan Kaya <okaya@codeaurora.org>
Cc: Tomasz Nowicki <tn@semihalf.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>