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>
diff --git a/board/mpl/mip405/Kconfig b/board/mpl/mip405/Kconfig
index 48ba91a..e003a43 100644
--- a/board/mpl/mip405/Kconfig
+++ b/board/mpl/mip405/Kconfig
@@ -1,4 +1,4 @@
-if TARGET_MIP405
+if TARGET_MIP405 || TARGET_MIP405T
 
 config SYS_BOARD
 	default "mip405"
@@ -9,4 +9,9 @@
 config SYS_CONFIG_NAME
 	default "MIP405"
 
+config ISO_STRING
+	string
+	default "MEV-10082-001" if TARGET_MIP405T
+	default "MEV-10072-001" if TARGET_MIP405
+
 endif