plat: rcar: Generate platform compatible string

Generate /compatible string for the platform, so that the subsequent
stages know which platform they are running on. This could be useful
when ie. building U-Boot that contains DTs for multiple platforms and
can thus decide on which platform it is running. This would ultimately
allow single bootloader binary for all Gen3 platforms.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>