xen/common: Move gic_dt_preinit() to common code

Introduce intc_dt_preinit() in the common codebase, as it is not
architecture-specific and can be reused by both PPC and RISC-V.
This function identifies the node with the interrupt-controller property
in the device tree and calls device_init() to handle architecture-specific
initialization of the interrupt controller.

Make minor adjustments compared to the original ARM implementation of
gic_dt_preinit():
 - Remove the local rc variable in gic_dt_preinit() since it is only used once.
 - Change the prefix from gic to intc to clarify that the function is not
   specific to ARM’s GIC, making it suitable for other architectures as well.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>