ARC: [arcfpga] Early ARC UART to be only activated by cmdline

Earlier CONFIG_SERIAL_ARC_CONSOLE would unconditionally enable the
legacy ARC UART based early console even if platform lacked that UART
(e.g. nSIM OSCI platform).

So a multi-platform image containing the 2 UART drivers won't work.

Fix that by probing the driver only if "earlyprintk=ttyARC0" is present
in cmdline.
Reported-by: Chuck Jordan <cjordan@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/boot/dts/angel4.dts

@@ -17,7 +17,7 @@ / {
 	interrupt-parent = <&intc>;
 
 	chosen {
-		bootargs = "console=ttyARC0,115200n8";
+		bootargs = "console=ttyARC0,115200n8 earlyprintk=ttyARC0";
 	};
 
 	aliases {

arch/arc/plat-arcfpga/platform.c

@@ -79,16 +79,11 @@ static void arc_fpga_serial_init(void)
 				   ARRAY_SIZE(fpga_early_devs));
 
 	/*
-	 * ARC console driver registers itself as an early platform driver
-	 * of class "earlyprintk".
-	 * Install it here, followed by probe of devices.
-	 * The installation here doesn't require earlyprintk in command line
-	 * To do so however, replace the lines below with
-	 *	parse_early_param();
-	 *	early_platform_driver_probe("earlyprintk", 1, 1);
-	 *						      ^^
+	 * ARC console driver registers (build time) as an early platform driver
+	 * of class "earlyprintk". However it needs explicit cmdline toggle
+	 * "earlyprintk=ttyARC0" to be successfuly runtime registered.
+	 * Otherwise the early probe below fails to find the driver
 	 */
-	early_platform_driver_register_all("earlyprintk");
 	early_platform_driver_probe("earlyprintk", 1, 0);
 
 	/*