reset: declare fdtdec_phandle_args as struct to fix warning

The of_xlate() callback needs to know fdtdec_phandle_args is struct.

Otherwise, the following warning is displayed.

include/reset-uclass.h:40:11: warning: 'struct fdtdec_phandle_args'
declared inside parameter list
    struct fdtdec_phandle_args *args);
           ^
include/reset-uclass.h:40:11: warning: its scope is only this
definition or declaration, which is probably not what you want

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>