MIPS: kconfig: add options for endianess select

Add new Kconfig option to let the user select the targets
endianess. Each target have to select SUPPORTS_BIG_ENDIAN
and/or SUPPORTS_LITTLE_ENDIAN to indicate which endianess
is supported.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
diff --git a/configs/dbau1500_defconfig b/configs/dbau1500_defconfig
index 5a02a78..d2fbff9 100644
--- a/configs/dbau1500_defconfig
+++ b/configs/dbau1500_defconfig
@@ -1,3 +1,4 @@
 CONFIG_SYS_EXTRA_OPTIONS="DBAU1500"
 CONFIG_MIPS=y
 CONFIG_TARGET_DBAU1X00=y
+CONFIG_SYS_BIG_ENDIAN=y