ARM: at91: fix warnings in pm.c

Fix a pair of missing statics on un-exported functions and
include <linux/platform_data/atmel.h> to provide the declaration
of at91_suspend_entering_slow_clock() to fix the following
sparse warnings:

arch/arm/mach-at91/pm.c:127:5: warning: symbol 'at91_suspend_entering_slow_clock' was not declared. Should it be static?
arch/arm/mach-at91/pm.c:358:6: warning: symbol 'at91rm9200_idle' was not declared. Should it be static?
arch/arm/mach-at91/pm.c:367:6: warning: symbol 'at91sam9_idle' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>