sunxi: fix SUNIV build when enabling D-Cache

The enable_caches function in architecture-specific board code is only
necessary for V7A CPUs, code for both V8A and ARM926 have already
declared this function.

Only provide our implementation of enable_caches() for V7A CPUs.

Signed-off-by: Icenowy Zheng <uwu@icenowy.me>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>