microblaze: Setup proper dependency for optimized lib functions

Optimized library functions are implemented only for little endian.
Setup this dependency via Kconfig.

commit 206d3642 ("arch/microblaze: add choice for endianness and
update Makefile") added an option to setup endianess via Kconfig.
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

arch/microblaze/Kconfig.platform

@@ -8,6 +8,7 @@ menu "Platform options"
 
 config OPT_LIB_FUNCTION
 	bool "Optimalized lib function"
+	depends on CPU_LITTLE_ENDIAN
 	default y
 	help
 	  Allows turn on optimalized library function (memcpy and memmove).