Change "intel,vm-map" property to be optional instead of required.

The driver implementation indicates it is not mandatory to have
"intel,vm-map" in the device tree:
 - probe doesn't fail in case it is absent.
 - explicit comment in code - "Incase intel,vm-map property is not
   defined, we assume incremental channel numbers".

Fixes: 748022ef ("hwmon: Add DT bindings schema for PVT controller")
Signed-off-by: Eliav Farber <farbere@amazon.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220908152449.35457-2-farbere@amazon.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>