arch: riscv: linker: do not require jedec,spi-nor to be 'okay'

It is not always possible to enable the jedec,spi-nor driver (e.g.
because the SPI controller can't be enabled). This is the case for
HiFive boards. Still, DT can be used for hardware description.

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