ns16550: add generic binding to unify the drivers

ns16550: add generic binding to unify the drivers

Add generic binding to unify ns16550 drivers. There are
several drivers using almost the same code, such as serial_dw,
serial_keystone, serial_omap, serial_ppc, serial_rockchip,
serial_tegra.c, and serial_x86. But each is platform specific.

The key difference between these drivers is the way to get
input clock frequency. With this unified approach, fixed clock
frequency should be extracted from "clock-frequency" property of
device tree blob. If this property is not available, the macro
CONFIG_SYS_NS16550_CLK will be used. It can be a constant or a
function to get clock, eg, get_serial_clock().

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>