docs/efi: Fix wrong compatible in dts example

docs/efi: Fix wrong compatible in dts example

The example in section "UEFI boot and dom0less on ARM" has a wrong
compatible for the DTB passthrough, it is "ramdisk" instead of
"device-tree".

This patch fixes the example.

Fixes: a1743fc3a9fe ("arm/efi: Use dom0less configuration when using EFI boot")
Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>
(cherry picked from commit 620ed2c8c777282154a91abca69083a40c9d918d)