This patch makes it possible for bt_ioremap() to be used before
paging_init(), via providing an early implementation of set_fixmap()
that can be used before paging_init().

This way boot_ioremap() can be replaced by bt_ioremap().
Signed-off-by: NHuang Ying <ying.huang@intel.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>