dma: dma_nxp_edma: drop the `hal-cfg-index` property

The HAL configuration binding can be done dynamically based on the
IP's address space. The `hal-cfg-index` property is more tied to
software rather than hardware so remove it as an attempt to clean
up the binding.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>