thermal: introduce thermal_zone_get_zone_by_name helper function

This patch adds a helper function to get a reference of
a thermal zone, based on the zone type name.

It will perform a zone name lookup and return a reference
to a thermal zone device that matches the name requested.
In case the zone is not found or when several zones match
same name or if the required parameters are invalid, it will return
the corresponding error code (ERR_PTR).

Cc: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Acked-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>