ARM: imx: avic: Move avic_saved_mask_reg under CONFIG_PM

ARM: imx: avic: Move avic_saved_mask_reg under CONFIG_PM

When building a kernel with CONFIG_PM undefined, the following warning happens:

arch/arm/mach-imx/avic.c:57:12: warning: 'avic_saved_mask_reg' defined but not used [-Wunused-variable]

Move avic_saved_mask_reg definition inside the '#ifdef CONFIG_PM' block to
avoid the warning.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>