boards: riscv: hifive: disable spi0 and flash0 by default

When flash connected to spi0 is used as ROM Flash, it can't be
instantiated. spi0 is in fact disabled now with CONFIG_SIFIVE_SPI_0_ROM,
which defaults to y. However, the child flash device is not. This causes
issues with tests relying on `jedec,spi-nor` because they can't find a
parent device.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>