PowerPC: Update MIP405/MIP405T to use Kconfig better

Convert CONFIG_MIP405T from SYS_EXTRA_OPTIONS to a real config

There are two boards, MIP405 and MIP405T that have a few differences.
Start by checking for CONFIG_TARGET_MIP405.  Then introduce
CONFIG_TARGET_MIP405T and use that not CONFIG_MIP405T.  Next, convert
also convert the usage of CONFIG_ISO_STRING to be based on Kconfig.

Signed-off-by: Tom Rini <trini@konsulko.com>