dt-bindings: dmaengine: nbpfaxi: Rename bindings documentation file

Rename the device tree bindings for renesas "Type-AXI" NBPFAXI* DMA
controllers from nbpfaxi.txt to renesas,nbpfaxi.txt.

This is part of an ongoing effort to name bindings documentation files for
Renesas IP blocks consistently, in line with the compat strings they
document.

Signed-off-by: Simon Horman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>