[ARM] Fix i.MX early serial port implementation.

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>
[DRIVERS] Fix i.MX serial driver.

There were a lot of interrupt source that could have been enabled by the
bootloader (u-boot) that were not disabled correctly.

Therefore it was possible to experiment a lot of unhandled interrupt that
would go on forever.

Also, we changed the way we detect data in the FIFO.

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>
[ARM] Add initial i.MX6 (Sabrelite) device tree

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>