serial/ns16550: add an option to avoid hanging on broken platforms

serial/ns16550: add an option to avoid hanging on broken platforms

Some platforms (e.g. IGEPv2 board) has a broken ns16550 UART that
does not set the TEMT bit when the transmitter is empty in SPL.
This makes U-Boot to hang while waiting for TEMT to be set.

Add a new option to avoid this:

CONFIG_SYS_NS16550_BROKEN_TEMT

16550 UART set the Transmitter Empty (TEMT) Bit when all output
has finished and the transmitter is totally empty. U-Boot waits
for this bit to be set to initialize the serial console. On some
broken platforms this bit is not set in SPL making U-Boot to
hang while waiting for TEMT. Define this option to avoid it.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>