thermal: rcar_gen3_thermal: record and check number of TSCs found

Record how many TSCs are found in struct rcar_gen3_thermal_priv, this is
needed to be able to add hardware interrupts for trip points later. Also
add a check to make sure at least one TSC is found.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>