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 <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20191213000657.931618-28-dima@arista.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/mxs-auart.c

@@ -12,10 +12,6 @@
  * 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>
@@ -1693,6 +1689,7 @@ static int mxs_auart_probe(struct platform_device *pdev)
 	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);