MIPS: ath25: Avoid undefined early_serial_setup() without SERIAL_8250_CONSOLE

MIPS: ath25: Avoid undefined early_serial_setup() without SERIAL_8250_CONSOLE

Currently MIPS allnoconfig with CONFIG_ATH25=y fails to link due to
missing support for early_serial_setup():

  LD      vmlinux
arch/mips/ath25/devices.o: In function ath25_serial_setup':
devices.c:(.init.text+0x68): undefined reference to 'early_serial_setup'

Rather than adding dependencies to the platform to force inclusion of
SERIAL_8250_CONSOLE together with it's dependencies like TTY, HAS_IOMEM,
etc, just make ath25_serial_setup() a no-op when the dependency is not
selected in the kernel config.

Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Cc: James Hogan <james.hogan@mips.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17700/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>