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/dbau1550_el_defconfig b/configs/dbau1550_el_defconfig
index 53b35ce..514f227 100644
--- a/configs/dbau1550_el_defconfig
+++ b/configs/dbau1550_el_defconfig
@@ -1,3 +1,4 @@
-CONFIG_SYS_EXTRA_OPTIONS="DBAU1550,SYS_LITTLE_ENDIAN"
+CONFIG_SYS_EXTRA_OPTIONS="DBAU1550"
 CONFIG_MIPS=y
 CONFIG_TARGET_DBAU1X00=y
+CONFIG_SYS_LITTLE_ENDIAN=y