unicore32: remove swiotlb support

unicore32 is a bog standard 32-bit port without larger physical address
space, highmem or any other obvious addressing limitation.  There should
be no need to bounce buffer using swiotlb.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Guan Xuetao <gxt@pku.edu.cn>