MIPS: AR7: Fix build failures when CONFIG_SERIAL_8250 is not enabled

This patch fixes the following build failure when CONFIG_SERIAL_8250
is not enabled in the kernel configuration:
arch/mips/ar7/built-in.o: In function 'ar7_register_devices':
platform.c:(.init.text+0x61c): undefined reference to 'early_serial_setup'
platform.c:(.init.text+0x61c): relocation truncated to fit: R_MIPS_26 against 'early_serial_setup'
platform.c:(.init.text+0x68c): undefined reference to 'early_serial_setup'
platform.c:(.init.text+0x68c): relocation truncated to fit: R_MIPS_26 against 'early_serial_setup'

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>