ar71xx: enable serial console on mikrotik devices

This patch enables the serial console on some Mikrotik devices (RB450,
RB450G, RB493G, RB750UP).

Signed-off-by: João Chaínho <joaochainho@gmail.com>

target/linux/ar71xx/patches-4.4/701-MIPS-ath79-add-routerboard-detection.patch

@@ -1,11 +1,14 @@
 --- a/arch/mips/ath79/prom.c
 +++ b/arch/mips/ath79/prom.c
-@@ -136,6 +136,13 @@ void __init prom_init(void)
+@@ -136,6 +136,16 @@ void __init prom_init(void)
  		initrd_end = initrd_start + fw_getenvl("initrd_size");
  	}
  #endif
 +
 +	if (strstr(arcs_cmdline, "board=750Gr3") ||
++	    strstr(arcs_cmdline, "board=750i") ||
++	    strstr(arcs_cmdline, "board=450") ||
++	    strstr(arcs_cmdline, "board=493") ||
 +	    strstr(arcs_cmdline, "board=951G") ||
 +	    strstr(arcs_cmdline, "board=2011L") ||
 +	    strstr(arcs_cmdline, "board=711Gr100") ||