ARM: imx: hide unused variable in #ifdef

A bugfix added a new local variable that is only used inside of an #ifdef
section, and unused if CONFIG_PERF_EVENTS is disabled:

arch/arm/mach-imx/mmdc.c:63:25: warning: 'cpuhp_mmdc_state' defined but not used [-Wunused-variable]

This moves the variable down inside that same ifdef.

Fixes: a051f220

 ("ARM/imx/mmcd: Fix broken cpu hotplug handling")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>