tty/serial: Migrate mxs-auart to use has_sysrq

The SUPPORT_SYSRQ ifdeffery is not nice as:
- May create misunderstanding about sizeof(struct uart_port) between
different objects
- Prevents moving functions from serial_core.h
- Reduces readability (well, it's ifdeffery - it's hard to follow)

In order to remove SUPPORT_SYSRQ, has_sysrq variable has been added.
Initialise it in driver's probe and remove ifdeffery.

Cc: Fabio Estevam <[e-mail]>
Cc: NXP Linux Team <[e-mail]>
Cc: Pengutronix Kernel Team <[e-mail]>
Cc: Shawn Guo <[e-mail]>
Cc: Sascha Hauer <[e-mail]>
Cc: [e-mail]
Signed-off-by: Dmitry Safonov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/mxs-auart.c

···
  * Copyright 2008 Embedded Alley Solutions, Inc All Rights Reserved.
  */
 
-#if defined(CONFIG_SERIAL_MXS_AUART_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/kernel.h>
 #include <linux/errno.h>
 #include <linux/init.h>
···
 	s->port.fifosize = MXS_AUART_FIFO_SIZE;
 	s->port.uartclk = clk_get_rate(s->clk);
 	s->port.type = PORT_IMX;
+	s->port.has_sysrq = IS_ENABLED(CONFIG_SERIAL_MXS_AUART_CONSOLE);
 
 	mxs_init_regs(s);