ARM: dts: renesas: blanche: Add FLASH node

Add a device node for the Spansion S29GL512S NOR FLASH on the Blanche
development board.  This FLASH resides in the external address space of
the Local Bus State Controller.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/7edb26bc29ad58198901ae6b6f869684473bb29d.1693481518.git.geert+renesas@glider.be