dt-bindings: Add support for the Broadcom UART driver

Add DT bindings for the Broadcom 8250 based UART driver. This
UART is based on an 8250 but adds additional functionality. The
additional features include the ability to use DMA for transfers and
a baud rate clock system that is more accurate at high baud rates.
This UART is backward compatible with the standard 8250 UART.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Al Cooper <alcooperx@gmail.com>
Link: https://lore.kernel.org/r/20210325185256.16156-2-alcooperx@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>