ns16550: Add compatible string for Raspberry Pi 4

ns16550: Add compatible string for Raspberry Pi 4

Per the BCM2835 peripherals datasheet [1] page 10:
"The UART core is build to emulate 16550 behaviour ... The implemented
UART is not a 16650 compatible UART However as far as possible the
first 8 control and status registers are laid out like a 16550 UART. Al
16550 register bits which are not supported can be written but will be
ignored and read back as 0. All control bits for simple UART receive/
transmit operations are available."

Additionally, Linux uses the 8250/16550 driver for the aux UART [2].

Unfortunately the brcm,bcm2835-aux-uart device tree binding doesn't
have the reg-shift and reg-io-width properties [3]. Thus, the reg-shift
and reg-io-width properties are inherent properties of this UART.

Thanks to Andre Przywara for contributing the reg-shift and
reg-io-width setting snippet.

In my testing, I have relied on enable_uart=1 being set in config.txt,
a configuration file read by the Raspberry Pi's firmware. With
enable_uart=1, the firmware performs UART initialization.

[1] https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2835/BCM2835-ARM-Peripherals.pdf
[2] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/tty/serial/8250/8250_bcm2835aux.c
[3] https://www.kernel.org/doc/Documentation/devicetree/bindings/serial/brcm,bcm2835-aux-uart.txt

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@dornerworks.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>