arm64/Kconfig: Don't set DMA_BOUNCE_UNALIGNED_KMALLOC

If enabled, DMA_BOUNCE_UNALIGNED_KMALLOC causes the swiotlb buffers
(64MB, by default) to be allocated, even on systems where the DMA
controller can reach all of RAM. This is a huge amount of RAM to
waste on a device with only 512MB to start with, such as the Zero 2 W.

See: https://github.com/raspberrypi/linux/issues/5975



Signed-off-by: Phil Elwell <phil@raspberrypi.com>