cache: StarFive: Require a 64-bit system

cache: StarFive: Require a 64-bit system

This has a bunch of {read,write}q() calls, so it won't work on 32-bit
systems.  I don't think there's any 32-bit StarFive systems, so for now
just require 64-bit.

Fixes: cabff60ca77d ("cache: Add StarFive StarLink cache management")
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Link: https://lore.kernel.org/r/20240722154519.25375-2-palmer@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>