uart: Add support for the Intel XScale controller

[bnovkov]

Summary

The ns8250 driver avoids clearing IER bit 0x10 to account for the
split "receiver time-out interrupt enable" bit, but it never sets
it in ier_rxbits even though a comment in ns8250_init implies so.

Fix this by setting IER_RXTMOUT if we've matched an XScale uart.

Event: BSDCan 2026

Test Plan

Tested on a Banana Pi BPI-F3 SoC. Using the console without the IER_RXTMOUT changes is impossible, applying this patch fixes it.