ARM: at91: select CONFIG_ARM_CPU_SUSPEND

The reference to cpu_resume requires the corresponding
generic code to be enabled when CONFIG_PM is set:

arch/arm/mach-at91/pm.o: In function `sama5d2_pm_init':
pm.c:(.init.text+0x5e8): undefined reference to `cpu_resume'

Fixes: 24a0f5c5 ("ARM: at91: pm: Add sama5d2 backup mode")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>