driver: cache-v5l2: Fix type casting warning on RV32

This patch fixes following warning for the riscv32 toolchain.

drivers/cache/cache-v5l2.c:122:16: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
  122 |         regs = (struct l2cache *)dev_read_addr(dev);
      |                ^

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>