remoteproc: imx: Broaden the Kconfig selection logic

Besides i.MX6SX and i.MX7D, there are other i.MX devices that contain
Cortex M4 and could make use of the imx remoteproc driver, such as
i.MX7ULP, i.MX8M, etc.

Instead of adding new SoC entries in the Kconfig logic, make it
broader by using the more generic ARCH_MXC, which encompasses
all the 32-bit and 64-bit i.MX devices.

Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>