ARM: imx: add missing iounmap()

Commit e76bdfd7 ("ARM: imx: Added perf functionality to mmdc driver")
introduced imx_mmdc_remove(), the mmdc_base need be unmapped in it if
config PERF_EVENTS is enabled.

If imx_mmdc_perf_init() fails, the mmdc_base also need be unmapped.

Fixes: e76bdfd7

 ("ARM: imx: Added perf functionality to mmdc driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>