Revert "MIPS: Add custom serial.h with BASE_BAUD override for generic kernel"

This reverts commit c8ec2041.

There's no more need to set BASE_BAUD to 0 to make earlycon work
properly on DTS-based boards since such cases were handled in commit
182ead3e ("earlycon: Remove hardcoded port->uartclk initialization
in of_setup_earlycon"). earlycon no longer initializes port->uartclk
with a value of BASE_BAUD * 16 when starting from FDT/OF.

Signed-off-by: Alexander Lobakin <alobakin@dlink.ru>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Microchip Linux Driver Support <UNGLinuxDriver@microchip.com>
Cc: Will Deacon <will@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Allison Randal <allison@lohutok.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org