thermal: rcar_gen3_thermal: check that TSC exists before memory allocation

Move the check for a TSC resource before allocating memory for a new
TSC. If no TSC is found there is little point in allocating memory for
it.

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>