doc: binding: rename directory ram to memory-controller

Alignment with kernel directory name as it have already bindings for
DDR controllers in the directory:
Documentation/devicetree/bindings/memory-controller

PS: the drivers using RAM u-class should be associated with
    this binding directory

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>

diff --git a/doc/device-tree-bindings/ram/k3-am654-ddrss.txt b/doc/device-tree-bindings/memory-controllers/k3-am654-ddrss.txt
similarity index 100%
rename from doc/device-tree-bindings/ram/k3-am654-ddrss.txt
rename to doc/device-tree-bindings/memory-controllers/k3-am654-ddrss.txt

diff --git a/doc/device-tree-bindings/ram/st,stm32-fmc.txt b/doc/device-tree-bindings/memory-controllers/st,stm32-fmc.txt
similarity index 100%
rename from doc/device-tree-bindings/ram/st,stm32-fmc.txt
rename to doc/device-tree-bindings/memory-controllers/st,stm32-fmc.txt

diff --git a/doc/device-tree-bindings/ram/st,stm32mp1-ddr.txt b/doc/device-tree-bindings/memory-controllers/st,stm32mp1-ddr.txt
similarity index 100%
rename from doc/device-tree-bindings/ram/st,stm32mp1-ddr.txt
rename to doc/device-tree-bindings/memory-controllers/st,stm32mp1-ddr.txt