thermal: qoriq: Add platform dependencies

The QorIQ Thermal Monitoring Unit is only present on Freescale E500MC
and Layerscape SoCs, and on NXP i.MX8 SoCs.  Add platform dependencies
to the QORIQ_THERMAL config symbol, to avoid asking the user about it
when configuring a kernel without support for any of the aforementioned
SoCs.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200507112955.23520-5-geert+renesas@glider.be