ARM: dts: STiH407: Move over to using the 'reserved-memory' API for obtaining DMA memory

Doing so saves quite a bit of code in the driver.

For more information on the 'reserved-memory' bindings see:

  Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt

Suggested-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>