dt-bindings: Rename file of DT bindings for Renesas memory controllers

For consistency with the naming of (most) other documentation files for DT
bindings for Renesas IP blocks rename the Renesas R-Mobile and SH-Mobile
memory controllers documentation file from renesas-memory-controllers.txt
to renesas,dbsc.txt.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>