serial: ns16550: Handle zero <clock-frequency> value

A working device tree node of ns16550 should never be populated
with value zero for the <clock-frequency> property. Unfortunately
this is the case for the QEMU ppce500 target.

Let's try to assign plat->clock to CONFIG_SYS_NS16550_CLK as the
last resort to handle such case.

This commit should be reverted when:

- The following QEMU patch [1] is merged, and
- U-Boot CI has upgraded its QEMU version that contains the fix

[1] http://patchwork.ozlabs.org/project/qemu-devel/patch/1612362288-22216-2-git-send-email-bmeng.cn@gmail.com/



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