thermal: qoriq: add multiple sensors support

The QorIQ Layerscape SoC has several thermal sensors but the current
driver only supports one.

Massage the code to be sensor oriented and allow the support for
multiple sensors.

Signed-off-by: Yuantian Tang <andy.tang@nxp.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>