CONFIG_SIFIVE_SPI_0_ROM (default y) was an option to disable spi0 if
used to access SPI Flash ROM. However, its design had a problem: it
relied on instance numbers. You had to set status okay for spi0 to make
it work (incongruent with the purpose of the option itself). This patch
makes things simpler: if such SPI0 is not available, simply keep it
disabled in DT. Bindings have been updated to mention this case.
Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
