ARM: imx: rework mx27_pm_init() call

mx27_pm_init() uses its own initcall, unlike all of the other
functions like it. Replacing the initcall with a .init_late()
callback makes imx27 more like the others and lets us remove
the last caller of cpu_is_mx27().

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>