riscv: move sifive_l2_cache.h to include/soc

riscv: move sifive_l2_cache.h to include/soc

The commit 9209fb51896f ("riscv: move sifive_l2_cache.c to drivers/soc")
moves the sifive L2 cache driver to driver/soc. It did not move the
header file along with the driver. Therefore this patch moves the header
file to driver/soc

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
[paul.walmsley@sifive.com: updated to fix the include guard]
Fixes: 9209fb51896f ("riscv: move sifive_l2_cache.c to drivers/soc")
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>