MIPS: SEAD3: Use generic ns16550a earlycon support

Stop selecting SYS_HAS_EARLY_PRINTK & remove the custom support for
early output to the ns16550a UARTs, instead relying upon generic
ns16550a earlycon support. This reduces the amount of platform code
required for SEAD3 without losing any functionality.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Jacek Anaszewski <j.anaszewski@samsung.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14049/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>