hw/char/imx_serial: Fix reset value of UFCR register

hw/char/imx_serial: Fix reset value of UFCR register

The value of the UCFR register is respected when echoing characters to the
terminal, but its reset value is reserved. Fix the reset value to the one
documented in the datasheet.

While at it move the related attribute out of the section of unimplemented
registers since its value is actually respected.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>