RISC-V: Use IS_ENABLED(CONFIG_CMODEL_MEDLOW)

IS_ENABLED should generally use CONFIG_ prefaced symbols and
it doesn't appear as if there is a CMODEL_MEDLOW define.

Signed-off-by: Joe Perches <joe@perches.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>