serial: ns16550: cleanups and 13x oversampling

This is a small patch set we've been using internally for a while now, and I figured it may be useful for mainline.

It originated from the desire of running the linux console at a higher bitrate, and therefore also the u-boot console for consistency's sake.

When using 16x oversampling and an 48 MHz reference clock, the standard rate 115200 requires a /26 divider, which also means that the only higher standard rate available is 230400 (/13 divider). In contrast, if 13x oversampling is used then the divider for 115200 is /32, which yields a much greater range of standard rates to be used.

This patch set consists of three cleanup patches related to TI OMAP and TI C6x style uarts, and finally one patch that configures 13x oversampling on both of them.

Matthijs van Duin