MLK-22417 imx8qxp_mek: Fix build break in fspi defconfig

MLK-22417 imx8qxp_mek: Fix build break in fspi defconfig

When CI_UDC DM driver is enabled in SPL, it must enable EHCI host. Then when
building SPL DM USB host, the usb_hub.c requires to enable SPL ENV.
The imx8qxp fspi config save ENV in MMC due to fspi can't write when running with M4.
But MMC driver is not enabled in fspi SPL, so the build was broken.
Fix the issue by adding CONFIG_SPL_ENV_IS_NOWHERE to all defconfig with SPL ENV
is enabled.

Signed-off-by: Ye Li <ye.li@nxp.com>