riscv: dts: sifive: use only generic JEDEC SPI NOR flash compatible

The compatible "issi,is25wp256" is undocumented and instead only a
generic jedec,spi-nor should be used (if appropriate).

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210920130248.145058-1-krzysztof.kozlowski@canonical.com