dt-bindings: hwmon: lm90: do not require VCC supply

dt-bindings: hwmon: lm90: do not require VCC supply

The LM90-like sensors usually need VCC supply, however this can be
hard-wired to the board main supply (like in SiFive HiFive Unmatched
RISC-V board).  Original bindings made VCC supply as required but in
practice several other boards skipped it.  Make it optional.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210920181913.338772-2-krzysztof.kozlowski@canonical.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>