powerpc: Clean up CHAIN_OF_TRUST related options

As things stand currently, there is only one PowerPC platform that
enables the options for CHAIN_OF_TRUST.  From the board header files,
remove a number of never-set options.  Remove board specific values from
arch/powerpc/include/asm/fsl_secure_boot.h as well.  Rework
include/config_fsl_chain_trust.h to not abuse the CONFIG namespace for
constructing CHAIN_BOOT_CMD.  Migrate all of the configurable addresses
to Kconfig.

If any platforms are re-introduced with secure boot support, everything
required should still be here, but now in Kconfig, or requires migration
of an option to Kconfig.

Cc: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>