ARM: mxs: fix memory leak in mxs_machine_init()

If of_property_read_string() failed, 'soc_dev_attr' should be
freed before return. Otherwise there is a memory leak.

Fixes: 2046338d ("ARM: mxs: Use soc bus infrastructure")
Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>