MIPS: loongson32: Remove dma.h and nand.h

Since commit 7b3415f5 ("MIPS: Loongson32: Remove
unused platform devices"), struct plat_ls1x_dma and plat_ls1x_nand
are unused. Then, dma.h and nand.h are useless.
Therefore, remove these useless header files.

Signed-off-by: Keguang Zhang <keguang.zhang@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>