of: Simplify of_dma_set_restricted_buffer() to use of_for_each_phandle()

Simplify of_dma_set_restricted_buffer() by using of_property_present()
and of_for_each_phandle() iterator.

Acked-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/20250423-dt-memory-region-v2-v2-2-2fbd6ebd3c88@kernel.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>