ARM: s3c64xx: avoid warning about 'struct device_node'

The change to simplify of_platform_populate() had an unintended
side-effect of introducing a build warning on s3c64xx:

In file included from arch/arm/mach-s3c64xx/mach-s3c64xx-dt.c:18:0:
arch/arm/mach-s3c64xx/common.h:27:30: error: 'struct device_node' declared inside parameter list will not be visible outside of this definition or declaration [-Werror]

This adds a forward-declaration for the structure name in the
header to avoid the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 850bea23

 ("arm: Remove unnecessary of_platform_populate with default match table")
Acked-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>