mxs: disable unused serial drivers

This disables PL010 and 8250 serial support for this platform
as both drivers are not used: the debug UART is PL011 and the
application UARTs use a dedicated MXS specific driver.

The kernel size reduction is neglectable, but it also removes
the non-working (confusing) /dev/ttyS0 and /dev/ttyS1 devices
in a running system (which generate errors when trying to use):

  root@OpenWrt:~# echo "hello world" > /dev/ttyS0
  ash: write error: I/O error
  root@OpenWrt:~#

Signed-off-by: Michael Heimpold <mhei@heimpold.de>