samples: jesd216: Exclude hifive_unleashed and unmatched boards

The HiFive Unleashed and HiFive Unmatched boards do not enable the
SPI0 controller that the flash is connected to. This test will not
build on these platforms because SPI0 bus controller is missing.

Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>