tty/serial: Migrate arc_uart 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: Vineet Gupta <[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/arc_uart.c

···
  *  -check if sysreq works
  */
 
-#if defined(CONFIG_SERIAL_ARC_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/module.h>
 #include <linux/serial.h>
 #include <linux/console.h>
···
 	port->flags = UPF_BOOT_AUTOCONF;
 	port->line = dev_id;
 	port->ops = &arc_serial_pops;
+	port->has_sysrq = IS_ENABLED(CONFIG_SERIAL_ARC_CONSOLE);
 
 	port->fifosize = ARC_UART_TX_FIFO_SIZE;