MX: serial_mxc: cleanup removing nasty #ifdef

The serial driver for iMX SOCs is continuosly changed if a
new SOC or not yet used port is used. CONFIG_SYS_<SOC>_<UART Port>
defines were used only to find the base address for the selected UART.
Instead of that, move the base address to the board configuration
file and drop all #ifdef from driver.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Marek Vasut <marek.vasut@gmail.com>
CC: Wolfgang Denk <wd@denx.de>
CC: Fabio Estevam <fabio.estevam@freescale.com>
CC: Helmut Raiger <helmut.raiger@hale.at>
CC: John Rigby <jcrigby@gmail.com>
CC: Matthias Weisser <weisserm@arcor.de>
CC: Jason Liu <jason.hui@linaro.org>
Acked-by: Jason Liu <jason.hui@linaro.org>