ARM: pxa: move extern declarations to pm.h

When CONFIG_IWMMXT is disabled, we get a warning in pxa3xx.c:

arch/arm/mach-pxa/pxa3xx.c: In function 'pxa3xx_cpu_pm_suspend':
arch/arm/mach-pxa/pxa3xx.c:109:2: error: ISO C90 forbids mixed declarations and code [-Werror=declaration-after-statement]

It turns out that there is an 'extern' declaration in the
middle of a function.

For consistency, this moves the declaration and two others from
the same file into pm.h.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>