serial: Kconfig: make MXC_UART usable for MX7 and IMX8M

i.MX7 and i.MX8M use mxc uart driver, so let's make the SoC could
use MXC_UART kconfig.
Signed-off-by: NPeng Fan <peng.fan@nxp.com>
Reviewed-by: NYe Li <ye.li@nxp.com>

drivers/serial/Kconfig

@@ -569,7 +569,7 @@ config MCFUART
 
 config MXC_UART
 	bool "IMX serial port support"
-	depends on MX5 || MX6
+	depends on MX5 || MX6 || MX7 || IMX8M
 	help
 	  If you have a machine based on a Motorola IMX CPU you
 	  can enable its onboard serial port by enabling this option.