MAINTAINERS: Add entry for Renesas R-Car thermal drivers

Add an entry to make myself a maintainer of the Renesas R-Car thermal
drivers.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Acked-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200216130252.125100-1-niklas.soderlund+renesas@ragnatech.se