[PATCH v6] tty: serial: add Freescale lpuart driver support

[Lu Jingchang-B35083]

>> This breaks the build on x86 systems with:
>>   CC [M]  drivers/tty/serial/fsl_lpuart.o
>> drivers/tty/serial/fsl_lpuart.c: In function ‘lpuart_probe’:
>> drivers/tty/serial/fsl_lpuart.c:751:2: error: implicit declaration of
>> function ‘of_alias_get_id’ [-Werror=implicit-function-declaration]
>>
>> so I can't take this :(
>
>On imx serial driver this build error is avoided by protecting the
>serial_imx_probe_dt() with "#ifdef CONFIG_OF" and you could so the same on
>the fsl_lpaurt driver to avoid the build issue that Greg reported.

[Lu Jingchang-B35083] 
  imx serial driver supports dt and non-dt uart device, "#ifdef CONFIG_OF" seems
to distinguish these, but fsl_lpuart only support dt device. 
  of_alias_get_id is defined in <linux/of.h>, it's referenced by <linux/of_device.h>
but with "#ifdef CONFIG_OF_DEVICE" restriction. So indirectly referencing "of.h" by
"of_device.h" without CONFIG_OF_DEVICE option will cause build error. Add direct
reference to "<linux/of.h>" in driver can resolve this error.
Thanks!