fhi_72: allow to provide dpdk_iova_mode to xran

In DPDK, IO Virtual Addresses (IOVA) refers to the memory addressing
scheme used for IO operations. The two IOVA modes supported by DPDK are:

- IOVA as Physical Address (PA): IO memory addresses correspond directly
  to physical addresses, no address translation required.
- IOVA as Virtual Address (VA): IO memory addresses align with user-space
  virtual addresses, requiring IOMMU to remap physical memory.

Previously, this was hardcoded to 0, to use IOVA as PA mode, with xRAN's
DPDK EAL initialization using the "--iova-mode=pa" flag.

This commit introduces a gNB configuration option to allow users to select
the desired IOVA mode. The default remains PA mode to
ensure backward compatibility.

Using IOVA VA mode allows DPDK to run without requiring SYS_ADMIN
capability in the docker container.