riscv: starfive: Add SYS_CACHE_SHIFT_6 to enable SYS_CACHELINE_SIZE

Some device driver need SYS_CACHELINE_SIZE macro. Add StarFive
SYS_CACHE_SHIFT_6 to enable it.

Signed-off-by: Minda Chen <minda.chen@starfivetech.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>