microblaze: __init_begin symbol must be aligned

The problem was that free_initmem pass to  free_initrd_mem got
bad aligned __init_begin symbol and free_initrd_mem don't care
about __init_end but take PAGE_SIZE instead.

Here is behavior in kernel bootlog.
ramdisk_execute_command from (init/main.c) was rewrite

Freeing unused kernel memory: 6224k freed
Failed to execute ��������������{���
Failed to execute ��������������{����.  Attempting defaults...
Mounting proc:
Mounting var:

Signed-off-by: Michal Simek <monstr@monstr.eu>