IIO: Add support for L3GD20H gyroscope

It can be used exactly like L3GD20 but it has a different WhoAmI
register value.

Signed-off-by: Robert Dolca <robert.dolca@intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>