ppc: Remove some SECURE_BOOT defconfigs

These specific configs are missing a number of migrations.  In addition,
they are blocking completion of the now-expired DM_MMC migration as it
requires enabling BLK.

Cc: Priyanka Jain <priyanka.jain@nxp.com>
Cc: Ruchika Gupta <ruchika.gupta@nxp.com>
Cc: Sumit Garg <sumit.garg@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>