ARM: imx: make imx51/3 suspend optional

A recent change to the imx53 power management caused a build
regression when CONFIG_SOC_IMX53 is disabled:

mach-imx/built-in.o:(.init.rodata+0x60): undefined reference to `imx53_suspend'
mach-imx/built-in.o:(.init.rodata+0x64): undefined reference to `imx53_suspend_sz'

This avoids the problem by compiling the code in question
conditionally on the presence of CONFIG_SOC_IMX53. For
consistency, I'm also changing the same thing for
CONFIG_SOC_IMX51.

An additional benefit of this approach is reduced code size
for kernels that only include support for one of the two
SoCs.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 1579c7b9 ("ARM: imx53: Set DDR pins to high impedance when in suspend to RAM.")
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>