drivers: ti: uart: Add TI specific 16550 initialization

On TI platforms the UART is disabled by default and must be explicitly
enabled using the MDR1 register.

NOTE: The original definition of
http://www.ti.com/lit/ds/symlink/pc16550d.pdf

 has no MDR register, but
many TI SoCs implementing 16550 do have a quirky MDR register
implemented. So, this should be enabled with TI_16550_MDR_QUIRK

NOTE: In such implementation, the CSR register does not exist.

Signed-off-by: Benjamin Fair <b-fair@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>