arm64: dts: r8a7795: add IMR-LX4 support

Describe the IMR-LX4 devices in the R8A7795 device tree.

Based on the original (and large) patch by Konstantin Kozhevnikov
<Konstantin.Kozhevnikov@cogentembedded.com>.

Signed-off-by: Konstantin Kozhevnikov <Konstantin.Kozhevnikov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>