ARM: shmobile: bockw reference dts: Add SPI FLASH

Add Spansion s25fl008k SPI FLASH and MTD partition, based on bockw legacy
board code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Tested-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>