microblaze: Enable IRQ in arch_cpu_idle

Microblaze requires to enable IRQ in cpu_idle loop.

It should be the part of this patch:
"microblaze: Use generic idle loop"
(sha1: e962bb9e9c)

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

arch/microblaze/kernel/process.c

@@ -160,3 +160,8 @@ int dump_fpu(struct pt_regs *regs, elf_fpregset_t *fpregs)
 	return 0; /* MicroBlaze has no separate FPU registers */
 }
 #endif /* CONFIG_MMU */
+
+void arch_cpu_idle(void)
+{
+       local_irq_enable();
+}