tty: serial: imx: add dependence and build for earlycon

Add the earlycon dependence and add earlycon Makefile support
to allow to build the driver.

Fixes: 699cc4df ("tty: serial: imx: add imx earlycon driver")
Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Link: https://lore.kernel.org/r/20200810060652.3436-1-fugang.duan@nxp.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/Kconfig

@@ -517,6 +517,7 @@ config SERIAL_IMX_CONSOLE
 
 config SERIAL_IMX_EARLYCON
 	bool "Earlycon on IMX serial port"
+	depends on ARCH_MXC || COMPILE_TEST
 	depends on OF
 	select SERIAL_EARLYCON
 	help

drivers/tty/serial/Makefile

@@ -43,6 +43,7 @@ obj-$(CONFIG_SERIAL_ZS) += zs.o
 obj-$(CONFIG_SERIAL_SH_SCI) += sh-sci.o
 obj-$(CONFIG_SERIAL_CPM) += cpm_uart/
 obj-$(CONFIG_SERIAL_IMX) += imx.o
+obj-$(CONFIG_SERIAL_IMX_EARLYCON) += imx_earlycon.o
 obj-$(CONFIG_SERIAL_MPC52xx) += mpc52xx_uart.o
 obj-$(CONFIG_SERIAL_ICOM) += icom.o
 obj-$(CONFIG_SERIAL_MESON) += meson_uart.o