MIPS: RB532: Avoid undefined early_serial_setup() without SERIAL_8250_CONSOLE

Currently MIPS allnoconfig with CONFIG_MIKROTIK_RB532=y fails to link due to
missing support for early_serial_setup():

  LD      vmlinux
arch/mips/rb532/serial.o: In function `setup_serial_port':
serial.c:(.init.text+0x14): 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 exclude arch/mips/rb532/serial.c from the build when it's
dependency is not selected in the kernel config.
Reported-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Cc: James Hogan <james.hogan@mips.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17701/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/rb532/Makefile

@@ -2,4 +2,6 @@
 # Makefile for the RB532 board specific parts of the kernel
 #
 
-obj-y	 += irq.o time.o setup.o serial.o prom.o gpio.o devices.o
+obj-$(CONFIG_SERIAL_8250_CONSOLE) += serial.o
+
+obj-y	 += irq.o time.o setup.o prom.o gpio.o devices.o