env: Kconfig: Add default option for PHY_CORTINA

Add PHY_CORTINA as default option in SYS_MMC_ENV_DEV Kconfig entry as
PHY_CORTINA require SYS_MMC_ENV_DEV value similar to FMAN_ENET or QE.
This helps in resolving compilation failure.

Signed-off-by: Kuldeep Singh <kuldeep.singh@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>