fsl: layerscape: guard *env_sf_get_env_addr() on CONFIG_ENV_IS_IN_SPI_FLASH

These functions can only be built and used when we have
ENV_IS_IN_SPI_FLASH, use that as a guard.

Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>