microblaze: Microblaze wants sys_fork for noMMU too

The patch "generic sys_fork / sys_vfork / sys_clone"
(sha1: d2125043aebf7f53cd1c72115c17b01d0bc06ce1)
introduced generic sys_fork with implementation for noMMU
which returns EINVAL for noMMU.

and the patch "microblaze: switch to generic fork/vfork/clone"
(sha1: f3268edbe6fe0ce56e62c6d6b14640aeb04864b7)
enable sys_fork only for MMU which is causing compilation
failure on noMMU system.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>