lib: utils/reset: use sbi_timer_mdelay() in gpio reset driver

We should use sbi_timer_mdelay() instead of custom gpio_mdelay() in
the gpio reset driver.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Xiang W <wxjstz@126.com>