cache: Make top level Kconfig menu a boolean dependent on RISCV

The next patch will add a new type of cache maintenance driver responsible
for flushing deeper than is necessary for non coherent DMA (current
use case of drivers/cache drivers), as needed when performing operations
such as memory hotplug and security unlocking of persistent memory. The two
types of operation are similar enough to share a drivers/cache directory
and MAINTAINERS but are otherwise currently unrelated.

To avoid confusion have two separate menus. Each has dependencies that are
implemented by making them boolean symbols, here CACHEMAINT_FOR_DMA
which is dependent on RISCV as all driver are currently for platforms of
that architecture. Set new symbol default to y to avoid breaking existing
configs. This has no affect on actual code built, just visibility of the
menu.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>