tty/serial: Migrate atmel_serial to use has_sysrq

tty/serial: Migrate atmel_serial 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.

While at it, remove forward-declaration of atmel_console - it wasn't
needed even at the moment the driver was first time introduced:
commit 3741cc97b8e7 ("[ARM] 3242/2: AT91RM9200 support for 2.6 (Serial)")

Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: Richard Genoud <richard.genoud@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20191213000657.931618-13-dima@arista.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>