[U-Boot] [RFC] Merge all ns16550 dm serial drivers into one

[Bin Meng]

Hi,

Currently there are 5 dm serial drivers, all of which are ns16550
compatible drivers. They are:

  serial_omap.c
  serial_dw.c
  serial_tegra.c
  serial_x86.c
  serial_ppc.c

All these drivers are pretty much similar. I think we can justmerge
these into one ns16550 driver.

If you think this is necessary, I will send a patch series to do this.

Regards,
Bin