tools/dma: move dma_map_benchmark from selftests to tools/dma

tools/dma: move dma_map_benchmark from selftests to tools/dma

dma_map_benchmark is a standalone developer tool rather than an
automated selftest. It has no pass/fail criteria, expects manual
invocation, and is built as a normal userspace binary. Move it to
tools/dma/ and add a minimal Makefile.

Suggested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Suggested-by: Barry Song <baohua@kernel.org>
Signed-off-by: Qinxin Xia <xiaqinxin@huawei.com>
Acked-by: Barry Song <baohua@kernel.org>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20251028120900.2265511-3-xiaqinxin@huawei.com