serial: earlycon: add DT support

This adds the infrastructure to generic earlycon for earlycon setup
using DT. The actual setup is not enabled until a following commit to
add the FDT parsing.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Arnd Bergmann <arnd@arndb.de>
Acked-by: Grant Likely <grant.likely@linaro.org>

include/asm-generic/vmlinux.lds.h

@@ -155,6 +155,7 @@
 #define CLK_OF_TABLES()		OF_TABLE(CONFIG_COMMON_CLK, clk)
 #define RESERVEDMEM_OF_TABLES()	OF_TABLE(CONFIG_OF_RESERVED_MEM, reservedmem)
 #define CPU_METHOD_OF_TABLES()	OF_TABLE(CONFIG_SMP, cpu_method)
+#define EARLYCON_OF_TABLES()	OF_TABLE(CONFIG_SERIAL_EARLYCON, earlycon)
 
 #define KERNEL_DTB()							\
 	STRUCT_ALIGN();							\
@@ -483,7 +484,8 @@
 	CLKSRC_OF_TABLES()						\
 	CPU_METHOD_OF_TABLES()						\
 	KERNEL_DTB()							\
-	IRQCHIP_OF_MATCH_TABLE()
+	IRQCHIP_OF_MATCH_TABLE()					\
+	EARLYCON_OF_TABLES()
 
 #define INIT_TEXT							\
 	*(.init.text)							\