firmware: use __SIZEOF_LONG__ for field offsets in fw_dynamic.h

The fields are of type unsigned long and are not pointers. While this
happens to be the same for RV32/RV64, it is not correct when compiling
for a CHERI-RISC-V system where pointers are twice the size of long.

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>