tty: serial: fsl_lpuart: Constify struct lpuart_soc_data

'struct lpuart_soc_data' are not modified in this driver.

Constifying these structures moves some data to a read-only section, so
increases overall security.

This also makes the code more consistent.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
 172668	  23470	    128	 196266	  2feaa	drivers/tty/serial/fsl_lpuart.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
 172924	  23214	    128	 196266	  2feaa	drivers/tty/serial/fsl_lpuart.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Sherry Sun <sherry.sun@nxp.com>
Link: https://lore.kernel.org/r/93dc860a06f92236db283c71be0640cc477b7291.1751092467.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>