[ARM] pxa: define zeus_power_off() only when CONFIG_PM enabled

Add some safety check for CONFIG_PM around zeus_power_off(). Without
it linking can fail like this:

arch/arm/mach-pxa/built-in.o: In function `zeus_power_off':
e800.c:(.text+0x2bc8): undefined reference to `pxa27x_cpu_suspend'

Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Acked-by: Marc Zyngier <maz@misterjones.org>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>