ARM: mxs: stub out mxs_pm_init for !CONFIG_PM

When building a kernel without CONFIG_PM, we get a link
error from referencing mxs_pm_init in the machine
descriptor. This defines a macro to NULL for that case.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>