x86: Use ePAPR defined properties for x86-uart

Use ePAPR defined properties for x86-uart: clock-frequency and
current-speed. Assign the value of clock-frequency in device tree
to plat->clock of x86-uart instead of using hardcoded number.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>