riscv: Fix arch_fixup_fdt always failing without /chosen

If /chosen was missing, chosen_offset would never get updated with the new
/chosen node. This would cause fdt_setprop_u32 to fail. This patch fixes
this by setting chosen_offset. In addition, log any errors from setting
boot-hartid as well.

Fixes: 5370478d ("riscv: Add boot hartid to device tree")
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>