ARM: imx: provide v7_cpu_resume() only on ARM_CPU_SUSPEND=y

512a928a ("ARM: imx: build v7_cpu_resume() unconditionally")
introduced an unintended linker error for i.MX6 configurations that have
ARM_CPU_SUSPEND=n which can happen if neither CONFIG_PM, CONFIG_CPU_IDLE,
nor ARM_PSCI_FW are selected.

Fix this by having v7_cpu_resume() compiled only when cpu_resume() it
calls is available as well.

The C declaration for the function remains unguarded to avoid future code
inadvertently using a stub and introducing a regression to the bug the
original commit fixed.

Cc: <stable@vger.kernel.org>
Fixes: 512a928a ("ARM: imx: build v7_cpu_resume() unconditionally")
Reported-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Tested-by: Roland Hieber <rhi@pengutronix.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>