soc: imx: Register SoC device only on i.MX boards

At the moment, using the ARM32 multi_v7_defconfig always results in two
SoCs being exposed in sysfs. This is wrong, as far as I'm aware the
Qualcomm DragonBoard 410c does not actually make use of a i.MX SoC. :)

  qcom-db410c:/sys/devices/soc0$ grep . *
  family:Freescale i.MX
  machine:Qualcomm Technologies, Inc. APQ 8016 SBC
  revision:0.0
  serial_number:0000000000000000
  soc_id:Unknown

  qcom-db410c:/sys/devices/soc1$ grep . *
  family:Snapdragon
  machine:APQ8016
  ...

This happens because imx_soc_device_init() registers the soc device
unconditionally, even when running on devices that do not make use of i.MX.
Arnd already reported this more than a year ago and even suggested a fix
similar to this commit, but for some reason it was never submitted.

Fix it by checking if the "__mxc_cpu_type" variable was actually
initialized by earlier platform code. On devices without i.MX it will
simply stay 0.

Cc: Peng Fan <peng.fan@nxp.com>
Fixes: d2199b34 ("ARM: imx: use device_initcall for imx_soc_device_init")
Reported-by: NArnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/CAK8P3a0hxO1TmK6oOMQ70AHSWJnP_CAq57YMOutrxkSYNjFeuw@mail.gmail.com/Signed-off-by: NStephan Gerhold <stephan@gerhold.net>
Reviewed-by: NFabio Estevam <festevam@gmail.com>
Reviewed-by: NPeng Fan <peng.fan@nxp.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

drivers/soc/imx/soc-imx.c

@@ -36,6 +36,10 @@ static int __init imx_soc_device_init(void)
 	int ret;
 	int i;
 
+	/* Return early if this is running on devices with different SoCs */
+	if (!__mxc_cpu_type)
+		return 0;
+
 	if (of_machine_is_compatible("fsl,ls1021a"))
 		return 0;