thermal: Kconfig: Update description for RCAR_GEN3_THERMAL config

The rcar_gen3_thermal driver also supports RZ/G2 SoC's, update the
description to reflect this.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200917152141.30070-1-prabhakar.mahadev-lad.rj@bp.renesas.com