sparc: remove obsolete config ARCH_ATU

Before consolidation of commit 4965a687 ("arch: define the
ARCH_DMA_ADDR_T_64BIT config symbol in lib/Kconfig"), the config ARCH_ATU
was used to control the state of the config ARCH_DMA_ADDR_T_64BIT. After
this consolidation, the config ARCH_ATU has been without use and effect.

Remove this obsolete config.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Link: https://lore.kernel.org/r/20231211083029.22078-1-lukas.bulwahn@gmail.com