rockchip: declear boot_devices in bootrom.h

boot_devices may defined in soc file, and used in board file,
we need to delear it in header file.
Signed-off-by: NKever Yang <kever.yang@rock-chips.com>

arch/arm/include/asm/arch-rockchip/bootrom.h

@@ -52,6 +52,8 @@ enum {
 	BROM_LAST_BOOTSOURCE = BROM_BOOTSOURCE_USB
 };
 
+extern const char * const boot_devices[BROM_LAST_BOOTSOURCE + 1];
+
 /**
  * Locations of the boot-device identifier in SRAM
  */