riscv: define a cache line size for the generic CPU

The USB 3.0 driver xhci-mem.c requires CONFIG_SYS_CACHELINE_SIZE to be set.

Define the cache line size for QEMU on RISC-V to be 64 bytes.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>