qemuboot: reduce default size of software I/O translation buffer

qemuboot: reduce default size of software I/O translation buffer

With 6.5+ (specifically, if DMA_BOUNCE_UNALIGNED_KMALLOC is set) the
SWIOTLB is used, and it defaults to 64MB. This is too much when there's
only 256MB of RAM, so request 0 slabs and lets the kernel round up to
the appropriate minimum (1MB on aarch64, typically). In virtual hardware
there's very little need for these bounce buffers, so the 64MB would be
mostly wasted.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>