LOONGSON_UART_BASE depends on EARLY_PRINTK || SERIAL_8250, but when
neither of these Kconfig symbols is set, the kernel build has errors:

../arch/mips/loongson2ef/common/serial.c: In function 'serial_init':
../arch/mips/loongson2ef/common/serial.c:66:25: error: 'loongson_uart_base' undeclared (first use in this function)
   66 |                         loongson_uart_base;
../arch/mips/loongson2ef/common/serial.c:66:25: note: each undeclared identifier is reported only once for each function it appears in
../arch/mips/loongson2ef/common/serial.c:68:41: error: '_loongson_uart_base' undeclared (first use in this function)
   68 |                         (void __iomem *)_loongson_uart_base;

Fix this by building serial.o only when one (or both) of these
Kconfig symbols is enabled.

Tested with:
	(a) EARLY_PRINTK=y, SERIAL_8250 not set;
	(b) EARLY_PRINTK=y, SERIAL_8250=y;
	(c) EARLY_PRINTK=y, SERIAL_8250=m.
	(d) EARLY_PRINTK not set, SERIAL_8250=y;
	(e) EARLY_PRINTK not set, SERIAL_8250=m;
	(f) EARLY_PRINTK not set, SERIAL_8250 not set.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>