arm64: dts: renesas: r8a7795-es1: add CSI-2 node

Renesas H3 ES1.0 have one extra CSI-2 node, CSI21 which is not present
for later ES versions of H3.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>