xen/arm: platform: fix Raspberry Pi compatible string

xen/arm: platform: fix Raspberry Pi compatible string

Both upstream [1] and downstream [2] Linux kernels use "brcm,bcm2711"
as the compatible string for Raspberry Pi 4. Add this string to our
platform compatible list.

The brcm,bcm2838 convention is abandoned. Remove it.

Rename the variables within the file to a rpi4_* prefix since the file
is meant to cover the Raspberry Pi 4 platform.

If you are using a device tree with the old compatible string
brcm,bcm2838, you will need to upgrade your device tree to one that has
the new brcm,bcm2711 compatible string.

[1] https://patchwork.kernel.org/patch/11165407/
[2] https://github.com/raspberrypi/linux/commit/53fdd7b8c8cb9c87190caab4fd459f89e1b4a7f8

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@dornerworks.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Release-acked-by: Juergen Gross <jgross@suse.com>