ARM: imx: mark imx53_suspend_sz as unused

ARM: imx: mark imx53_suspend_sz as unused

Unused 'static const' variables cause a warning when building with
W=1, and imx53_suspend_sz has a definition for this as an
alternative when CONFIG_SUSPEND is disabled:

    In file included from arch/arm/mach-imx/cpu.c:9:
    arch/arm/mach-imx/common.h:101:18: error: 'imx53_suspend_sz' defined but not used [-Werror=unused-const-variable=]

It's still referenced though, so mark it as __maybe_unused, so
the one user can address the dummy copy and other files that include
the header don't produce a warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>