boards: renesas: dts: Add overlay files to support the MEMC

In order to avoid defining almost the same overlays in the available
sample codes, tests and user applications, a common overlay file
per memory type is demonstrated under the boards dts folder.
Currently only the PSRAM interface is supported and the APS6404L
PSRAM QSPI memory device is demonstrated. In doing so, an
application code will only have to define another overlay file explicitly,
under application's board folder, to overwrite the default QSPI
controller's settings. In either case, users should explicitly
invoke the requested overlay files  at 'west build' invokation
via the DTC_OVERLAY_FILE system variable.

Signed-off-by: Ioannis Karachalios <ioannis.karachalios.px@renesas.com>