firmware: imx: IMX_SCMI_MISC_DRV should depend on ARCH_MXC

The i.MX System Controller Management Interface firmware is only present
on Freescale i.MX SoCs.  Hence add a dependency on ARCH_MXC, to prevent
asking the user about this driver when configuring a kernel without
Freescale i.MX platform support.

Fixes: 514b2262 ("firmware: arm_scmi: Fix i.MX build dependency")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>