i.MX use u-boot baud rate and don't assume UART master clock

1) Change the i.MX serial driver to use the baud rate set in the
 u-boot environment

2) don't assume a 16MHz value for PERCLK1 in baud rate calculations

3) don't write a 1 to the RDR bit in the USR2 reg. (bit is not "write
 one to clear" like other status bits in the reg.)

Signed-off-by: Andrew Dyer <adyer@righthandtech.com>