imx9: Improve boot mode autodetection

imx9: Improve boot mode autodetection

Improve "mmcautodetect=yes" boot mode autodetection to able to use it
if CONFIG_ENV_IS_NOWHERE=y is used for i.MX9 SoCs and i.MX93 EVK board.

If both CONFIG_ENV_IS_IN_MMC=y and CONFIG_ENV_IS_NOWHERE=y are in the
defconfig, CONFIG_ENV_IS_IN_MMC=y will be overiden default
CONFIG_ENV_IS_NOWHERE settings.

Goal is in this patch to able to use the boot mode autodetection
if defconfig use only CONFIG_ENV_IS_NOWHERE=y option
(without CONFIG_ENV_IS_IN_MMC) for any i.MX9 SoC.

Signed-off-by: Benjamin Szőke <egyszeregy@freemail.hu>