serial: 8250_uniphier: add UniPhier serial driver

Add the driver for on-chip UART used on UniPhier SoCs.

This hardware is similar to 8250, but the register mapping is
slightly different:
  - The offset to FCR, MCR is different.
  - The divisor latch access bit does not exist.  Instead, the
    divisor latch register is available at offset 9.

This driver overrides serial_{in,out}, dl_{read,write} callbacks,
but wants to borrow most of code from 8250_core.c.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>