nds32: include <asm/arch-*/*.h> instead of <asm/arch/*.h>

There are only two SoC-specific headers for this architecture:
 - arch/nds32/include/asm/arch-ag101/ag101.h
 - arch/nds32/include/asm/arch-ag102/ag102.h

Those two have different file names, so there is no advantage to
include them via symbolic linked directory.

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