platform: generic-arm64: Adjust hypervisor_mmio_map_region error handling

On qemu-generic-arm64-gicv3-* builds, hypervisor_mmio_map_region may
unexpectedly return ERR_INVALID_ARGS instead of ERR_NOT_SUPPORTED which
prevents proper UART initialization and blocks console output from the
guest VM. Until this can be root caused, keep going if we get that
error.

Bug: 375584439
Test: Boot trusty in QEMU w/o pvmfw and verify that UART works with
      MMIO_GUARD_ENABLED=true
Change-Id: I89044615eedf4cbd38a45fb058c79af047f6affd