utils/reset: Remove fdt_reset_thead

In the past, we used fdt_reset_thead to help customers with prototype
verification. However, with the emergence of the Big-little SoC system,
it can no longer meet the demand. Therefore, we use zero_stage_boot
instead of fdt_reset_thead. It cleans up the opensbi code and ends the
disputation of reset_sample's dts.

This patch removes the fdt_reset_thead component and updates the related
doc.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Anup Patel <anup@brainfault.org>