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.