Documentation: devicetree: mtd: add w25q64 to list of supported SPI flashes

W25Q64 is found on TI's AM335x ICEv2 board. Add it to list
for supported SPI flash devices. This flash can be identified
using JEDEC READ ID opcode.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>