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/qemu_mips64_defconfig b/configs/qemu_mips64_defconfig
index 2948355..36897b9 100644
--- a/configs/qemu_mips64_defconfig
+++ b/configs/qemu_mips64_defconfig
@@ -1,3 +1,3 @@
-CONFIG_SYS_EXTRA_OPTIONS="SYS_BIG_ENDIAN"
 CONFIG_MIPS=y
 CONFIG_TARGET_QEMU_MIPS64=y
+CONFIG_SYS_BIG_ENDIAN=y