serial-howto: remove altera_jtag_uart and altera_uart from the list

Since both altera_jtag_uart and altera_uart are converted to driver
model, remove them from the list of drivers remaining to convert.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Chin Liang See <clsee@altera.com>