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>

target/linux/mxs/config-4.14

@@ -262,9 +262,7 @@ CONFIG_RTC_MC146818_LIB=y
 CONFIG_RWSEM_XCHGADD_ALGORITHM=y
 # CONFIG_SCHED_INFO is not set
 # CONFIG_SCSI_DMA is not set
-CONFIG_SERIAL_8250_FSL=y
-CONFIG_SERIAL_AMBA_PL010=y
-CONFIG_SERIAL_AMBA_PL010_CONSOLE=y
+# CONFIG_SERIAL_8250 is not set
 CONFIG_SERIAL_AMBA_PL011=y
 CONFIG_SERIAL_AMBA_PL011_CONSOLE=y
 CONFIG_SERIAL_MCTRL_GPIO=y