ar71xx: enable serial console on Mikrotik RB411/RB433

This patch adds support for serial console on Mikrotik RB411 and RB433
series devices.

Signed-off-by: David Varga <duvi@duvinet.hu>
[picked from FS#377]
Signed-off-by: Mathias Kresin <dev@kresin.me>

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

@@ -1,12 +1,14 @@
 --- a/arch/mips/ath79/prom.c
 +++ b/arch/mips/ath79/prom.c
-@@ -136,6 +136,16 @@ void __init prom_init(void)
+@@ -136,6 +136,18 @@ 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=411") ||
++	    strstr(arcs_cmdline, "board=433") ||
 +	    strstr(arcs_cmdline, "board=450") ||
 +	    strstr(arcs_cmdline, "board=493") ||
 +	    strstr(arcs_cmdline, "board=951G") ||