powerpc: Update a BKL related comment

The commit 5e3d20a6 remove bkl from startup code so setup_arch() it isn't called
with bkl held anymore. Update the comment on top of that function.
Fix also a typo.

This work was supported by a hardware donation from the CE Linux Forum.
Signed-off-by: NAlessio Igor Bogani <abogani@texware.it>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/setup_64.c

@@ -497,9 +497,8 @@ static void __init emergency_stack_init(void)
 }
 
 /*
- * Called into from start_kernel, after lock_kernel has been called.
- * Initializes bootmem, which is unsed to manage page allocation until
- * mem_init is called.
+ * Called into from start_kernel this initializes bootmem, which is used
+ * to manage page allocation until mem_init is called.
  */
 void __init setup_arch(char **cmdline_p)
 {