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: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>