thermal: core: Constify static attribute_group structs

The only usage of these structs is to assign their address to the
thermal_zone_attribute_groups array, which consists of pointers to
const, so make them const to allow the compiler to put them in read-only
memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201128234342.36684-1-rikard.falkeborn@gmail.com