doc: intel: Update serial driver changes in slimbootloader.rst

Now, Slim Bootloader uses NS16550_DYNAMIC to support serial port
configuration at runtime, so no more code change is required.
Therefore, remove unnecessary steps and fix minor typo.

Signed-off-by: Aiden Park <aiden.park@intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>